Add tests to make sure different preconditions affect each other

Author: tornado80

tests/preconditions-do-not-mess-up-bad.slang

@@ -0,0 +1,12 @@
+method a(n: Int)
+    requires n >= 0
+{
+    assert true
+}
+
+method b(n: Int)
+    requires n < 0
+{
+    // @CheckError
+    assert false
+}

tests/preconditions-do-not-mess-up-good.slang

@@ -0,0 +1,11 @@
+method a(n: Int)
+    requires n >= 0
+{
+    assert true
+}
+
+method b(n: Int)
+    requires n < 0
+{
+    assert true
+}