Generate buildbot json tests: Split mixin tests

This CL moves tests for the mixins feature into its own suite. It also
moves the test data inputs for those tests near this.

Change-Id: Id0e2c97ee59497e239231cc53e300d2f1f655b5c
Reviewed-on: https://chromium-review.googlesource.com/c/1303046Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603568}

testing/buildbot/generate_buildbot_json_coveragetest.py

@@ -19,8 +19,8 @@ def main():
   cov = coverage.coverage(include='*generate_buildbot_json.py')
   cov.start()
   import generate_buildbot_json_unittest
-  suite = unittest.TestLoader().loadTestsFromTestCase(
-    generate_buildbot_json_unittest.UnitTest)
+  suite = unittest.TestLoader().loadTestsFromModule(
+    generate_buildbot_json_unittest)
   unittest.TextTestRunner(stream=FakeStream()).run(suite)
   cov.stop()
   outf = cStringIO.StringIO()