gce_au_worker: Fix unittest

gce_au_worker_unittest.py was broken by CL:315816. The particular broken test
is not optimal as it replies on some implementation detail, but I can't think of
an _easy_ alternative either.

TEST=python au_test_harness/gce_au_worker_unittest.py passes

Change-Id: Iff89ba0a3a8b4751a20d0df93a3d4097829988b8
Reviewed-on: https://chromium-review.googlesource.com/317326
Commit-Ready: Daniel Wang <wonderfly@google.com>
Tested-by: Daniel Wang <wonderfly@google.com>
Reviewed-by: Daniel Wang <wonderfly@google.com>
Reviewed-by: Andrey Ulanov <andreyu@google.com>
diff --git a/au_test_harness/gce_au_worker_unittest.py b/au_test_harness/gce_au_worker_unittest.py
index 9d8f79b..3820a71 100755
--- a/au_test_harness/gce_au_worker_unittest.py
+++ b/au_test_harness/gce_au_worker_unittest.py
@@ -209,8 +209,11 @@
         return ''
     def _OverrideRunCommand(cmd, *unused_args, **unused_kwargs):
-      remote = cmd[-3]
-      test = cmd[-2]
+      """A mock of cros_build_lib.RunCommand that injects arguments."""
+      # In this test setup, |test| and |remote| should be the third and fourth
+      # last argument respectively.
+      test = cmd[-3]
+      remote = cmd[-4]
       actual_tests_run.append(dict(remote=remote, test=test))
       return cros_build_lib.CommandResult()