[test] Limit "unittests" suite to default variant.

This changes the unittests suite to be run in only the default variant,
because unit tests are (and should be) written to be independent from
global flags.

R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/1067863007

Cr-Commit-Position: refs/heads/master@{#28054}

tools/testrunner/local/testsuite.py

@@ -272,5 +272,8 @@ class GoogleTestSuite(TestSuite):
             ["--gtest_print_time=0"] +
             context.mode_flags)
 
+  def VariantFlags(self, testcase, default_flags):
+    return [[]]
+
   def shell(self):
     return self.name