Add bad test for early return

Author: tornado80

tests/early-return-bad.slang

@@ -0,0 +1,11 @@
+method M(n: Int) : Int
+  ensures n == 1 ==> result == 2
+  ensures n != 1 ==> result == 3
+{
+  match {
+    n == 1 =>
+        // @CheckError
+        return 9
+  };
+  return 3
+}