Allow overridding "app_name" for "test" template on iOS.

testing/test.gni

Index: testing/test.gni
diff --git a/testing/test.gni b/testing/test.gni
index bfebe5ffecee18a153fe9708b86fd53e91947ef7..53e84153aebdaacfb201d3401645399db0ec4c9d 100644
--- a/testing/test.gni
+++ b/testing/test.gni
@@ -295,10 +295,18 @@ template("test") {
       # TODO: Make this configurable and only provide a default that can be
       # overridden.
       info_plist = "//testing/gtest_ios/unittest-Info.plist"
-      app_name = target_name
       entitlements_path = "//testing/gtest_ios"
       code_signing_identity = ""
       testonly = true
+
+      # TODO(crbug.com/603102): remove this once gyp support is dropped and all
+      # application uses the target name as value for BUNDLE_ID_TEST_NAME.
+      if (defined(invoker.app_name)) {
+        app_name = invoker.app_name
+      } else {
+        app_name = target_name
+      }
+
       extra_substitutions = [ "BUNDLE_ID_TEST_NAME=$app_name" ]
 
       # See above call.