[autotest] Use existing config value override functionality in unit tests

I had hand-rolled a way to mock out fetching config values in
dynamic_suite_unittest.py, but it turns out such a thing already exists.

BUG=chromium-os:26466
TEST=unit tests.
STATUS=Fixed

Change-Id: Ib7f3f188a82ac6665088d3540ff3a44e7ba3871a
Reviewed-on: https://gerrit.chromium.org/gerrit/16300
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Scott Zawalski <scottz@chromium.org>
Commit-Ready: Chris Masone <cmasone@chromium.org>
diff --git a/server/cros/dynamic_suite_unittest.py b/server/cros/dynamic_suite_unittest.py
index 73f6d5f..2f8e88b 100755
--- a/server/cros/dynamic_suite_unittest.py
+++ b/server/cros/dynamic_suite_unittest.py
@@ -153,10 +153,9 @@
         cf_getter.get_control_file_contents_by_name('autoupdate').AndReturn('')
         self.reimager._cf_getter = cf_getter
 
-        # Fake out getting the image URL pattern.
-        self.mox.StubOutWithMock(dynamic_suite, '_image_url_pattern')
-        dynamic_suite._image_url_pattern().AndReturn(self._URL)
-
+        self._CONFIG.override_config_value('CROS',
+                                           'image_url_pattern',
+                                           self._URL)
         self.afe.create_job(
             control_file=mox.And(mox.StrContains(self._NAME),
                                  mox.StrContains(self._URL % self._NAME)),