Add check for $experimental_testsupport in isExperimentalJavaTest.

This should be a no-op because because before testsupport is set (https://github.com/bazelbuild/bazel/blob/3d362fb9a122ceee6d781be127dfedbbff8051f8/src/main/java/com/google/devtools/build/lib/bazel/rules/java/BazelJavaSemantics.java#L509), it's checking that main_class is pointing to the experimental testrunner.

This is needed because android_local_test doesn't have a main_class attribute and then bazel breaks when it tries to get the main_class attribute.

RELNOTES: None
PiperOrigin-RevId: 178964286

src/main/java/com/google/devtools/build/lib/bazel/rules/java/BazelJavaSemantics.java

@@ -158,7 +158,8 @@ private String getMainClassFromRule(RuleContext ruleContext) {
   }
 
   private boolean isExperimentalJavaTest(RuleContext ruleContext) {
-    return TargetUtils.isTestRule(ruleContext.getRule())
+    return ruleContext.attributes().has("$experimental_testsupport")
+        && TargetUtils.isTestRule(ruleContext.getRule())
         && getMainClassFromRule(ruleContext).equals(EXPERIMENTAL_TEST_RUNNER_MAIN_CLASS);
   }