kotlin: Adding bazel macro for more convenient test target decla… (#221)

Creating test targets for Kotlin is a bit tedious. This change is to help alleviate that.

Some callouts:
1. This will *only* run tests within the `io.envoyproxy.envoymobile` package space
2. This will fail if there are no tests in the provided test target

I feel these are reasonable requirements for this project and can rely on this for the near future (we can revisit when we there is a solution which is more official).

Bazel out of the box:
```
load("@io_bazel_rules_kotlin//kotlin:kotlin.bzl", "kt_jvm_test")

kt_jvm_test(
    name = "example_kotlin_test",
    test_target = "com.example.myproject.mymodule.ExampleTest"
    srcs = [
        "ExampleTest.kt",
    ],
    deps = [
        "//:another_target",
        "@maven//:org_assertj_assertj_core",
        "@maven//:junit_junit",
    ]
)
```

With this change:
```
load("//bazel:kotlin_test.bzl", "envoy_mobile_kt_test)

envoy_mobile_kt_test(
    name = "example_kotlin_test",
    srcs = [
        "ExampleTest.kt",
    ],
    deps = [
        "//:another_target",
    ]
)
```

Took this solution from: https://stackoverflow.com/questions/46365464/how-to-run-all-tests-in-bazel-from-a-single-java-test-rule

For an explanation of how to fill out the fields, please see the relevant section
in [PULL_REQUESTS.md](https://github.com/envoyproxy/envoy/blob/master/PULL_REQUESTS.md)

Description: Adding bazel macro for more convenient test target declaration
Risk Level: low
Testing: locally
Docs Changes: n/a
Release Notes:
[Optional Fixes #Issue]
[Optional Deprecated:]

Signed-off-by: JP Simard <jp@jpsim.com>

mobile/WORKSPACE

@@ -102,6 +102,20 @@ git_repository(
     shallow_since = "1552938175 -0400",
 )
 
+load("@rules_jvm_external//:defs.bzl", "maven_install")
+
+maven_install(
+    artifacts = [
+        # Test artifacts
+        "org.assertj:assertj-core:3.9.0",
+        "junit:junit:4.12"
+    ],
+    repositories = [
+        "https://repo1.maven.org/maven2",
+        "https://jcenter.bintray.com/",
+    ]
+)
+
 # Bazel Kotlin 1.3 patch: https://github.com/bazelbuild/rules_kotlin/issues/159
 # bazelbuild/rules_kotlin currently doesn't work with Kotlin 1.3
 # TODO: https://github.com/lyft/envoy-mobile/issues/68

mobile/bazel/BUILD

@@ -1 +1,17 @@
 licenses(["notice"])  # Apache 2
+
+load("@envoy//bazel:envoy_build_system.bzl", "envoy_package")
+load("@io_bazel_rules_kotlin//kotlin:kotlin.bzl", "kt_jvm_library")
+
+envoy_package()
+
+kt_jvm_library(
+    name = "envoy_mobile_test_suite",
+    srcs = [
+        "EnvoyMobileTestSuite.kt",
+    ],
+    visibility = ["//visibility:public"],
+    deps = [
+        "@maven//:junit_junit",
+    ],
+)

mobile/bazel/EnvoyMobileTestSuite.kt

@@ -0,0 +1,69 @@
+package io.envoyproxy.envoymobile.bazel
+
+import junit.framework.JUnit4TestAdapter
+import junit.framework.TestSuite
+import org.junit.runner.RunWith
+import java.io.File
+import java.lang.RuntimeException
+import java.net.URLClassLoader
+import java.util.zip.ZipFile
+
+/**
+ * This is class is taken from https://stackoverflow.com/questions/46365464/how-to-run-all-tests-in-bazel-from-a-single-java-test-rule
+ *
+ * Translated to Kotlin and slightly modified
+ *
+ * Requirements:
+ * 1. This only allows tests to be run if the package is within `io.envoyproxy.envoymobile`
+ * 2. This requires at least one test to be defined in the test target
+ */
+@RunWith(org.junit.runners.AllTests::class)
+object EnvoyMobileTestSuite {
+  private const val CLASS_SUFFIX = ".class"
+  private const val ENVOY_MOBILE_PACKAGE = "io.envoyproxy.envoymobile"
+
+  @JvmStatic
+  fun suite(): TestSuite {
+    val suite = TestSuite()
+    val classLoader = Thread.currentThread().contextClassLoader as URLClassLoader
+
+    val testAdapters = mutableListOf<JUnit4TestAdapter>()
+    // The first entry on the classpath contains the srcs from java_test
+    val classesInJar = findClassesInJar(File(classLoader.urLs[0].path))
+    for (clazz in classesInJar) {
+      val name = Class.forName(clazz)
+      val testAdapter = JUnit4TestAdapter(name)
+      testAdapters.add(testAdapter)
+    }
+
+    if (testAdapters.isEmpty()) {
+      throw RuntimeException("Unable to find any tests in test target")
+    }
+
+    for (testAdapter in testAdapters) {
+      suite.addTest(testAdapter)
+    }
+    return suite
+  }
+
+  private fun findClassesInJar(jarFile: File): Set<String> {
+    val classNames = mutableSetOf<String>()
+
+    ZipFile(jarFile).use { zipFile ->
+      val entries = zipFile.entries()
+      for (entry in entries) {
+        val entryName = entry.name
+
+        if (entryName.endsWith(CLASS_SUFFIX)) {
+          val classNameEnd = entryName.length - CLASS_SUFFIX.length
+          val resolvedClass = entryName.substring(0, classNameEnd).replace('/', '.')
+          if (resolvedClass.contains(ENVOY_MOBILE_PACKAGE)) {
+            classNames.add(resolvedClass)
+          }
+        }
+      }
+    }
+
+    return classNames
+  }
+}

mobile/bazel/kotlin_test.bzl

@@ -0,0 +1,30 @@
+load("@io_bazel_rules_kotlin//kotlin:kotlin.bzl", "kt_jvm_test")
+
+# A basic macro to make it easier to declare and run kotlin tests
+#
+# Ergonomic improvements include:
+# 1. Avoiding the need to declare the test_class which requires a fully qualified class name (example below)
+# 2. Avoiding the need to redeclare common unit testing dependencies like JUnit
+# 3. Ability to run more than one test file per target
+#
+# Usage example:
+# load("//bazel:kotlin_test.bzl", "envoy_mobile_kt_test)
+#
+# envoy_mobile_kt_test(
+#     name = "example_kotlin_test",
+#     srcs = [
+#         "ExampleTest.kt",
+#     ],
+# )
+#
+def envoy_mobile_kt_test(name, srcs, deps = []):
+    kt_jvm_test(
+        name = name,
+        test_class = "io.envoyproxy.envoymobile.bazel.EnvoyMobileTestSuite",
+        srcs = srcs,
+        deps = deps + [
+            "//bazel:envoy_mobile_test_suite",
+            "@maven//:org_assertj_assertj_core",
+            "@maven//:junit_junit",
+        ],
+    )