fix(tests): update mock return value in TrialBuildMainTest to use a dictionary

Author: hunsche

infra/build/functions/trial_build_test.py

@@ -73,7 +73,7 @@ def test_build_steps_correct(self, mock_gcb_build_and_push_images,
     del mock_wait_on_builds
     self.maxDiff = None  # pylint: disable=invalid-name
     build_id = 1
-    mock_run_build.return_value = build_id
+    mock_run_build.return_value = {'id': build_id}
     branch_name = 'mybranch'
     project = 'skcms'
     args = [