chromite: Report perf values from image test stage

Image tests can call OutputPerfValues during their runs. The stage will
then aggregate all recorded perf values and report them to chromeperf.

lib/perf_dashboard_config.json is a list of dictionaries of known
performance tests:

[
  {
    'master_name': 'ChromeOSPerf',  # Master in ChromePerf
    'test_name': 'image_test.ClassName.TestMethodName'  # The test
  },
  ...
]

This CL also removes BuilderRunMock from general_stages_unittest and
give ImageTestStage option and config name 'image_test'.

BUG=chromium:386198
BUG=chromium:394076
TEST=unittest
TEST=trybot on "release" group

Change-Id: I65895f95d0c9b0402db36a8e72c1dfc6878547b5
Reviewed-on: https://chromium-review.googlesource.com/207818
Reviewed-by: Nam Nguyen <namnguyen@chromium.org>
Commit-Queue: Nam Nguyen <namnguyen@chromium.org>
Tested-by: Nam Nguyen <namnguyen@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index b13593a..f32b96f 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -1262,6 +1262,10 @@
                           default=True,
                           help=('Override values from buildconfig and run no '
                                 'tests.'))
+  group.add_remote_option('--noimagetests', action='store_false',
+                          dest='image_test', default=True,
+                          help=('Override values from buildconfig and run no '
+                                'image tests.'))
   group.add_remote_option('--nouprev', action='store_false', dest='uprev',
                           default=True,
                           help=('Override values from buildconfig and never '