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

tools/testrunner/local/testsuite.py

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 254 matching lines @@
         tests.append(test)
     tests.sort()
     return tests
 
   def GetFlagsForTestCase(self, testcase, context):
     return (testcase.flags + ["--gtest_filter=" + testcase.path] +
             ["--gtest_random_seed=%s" % context.random_seed] +
             ["--gtest_print_time=0"] +
             context.mode_flags)
 
+  def VariantFlags(self, testcase, default_flags):
+    return [[]]
+
   def shell(self):
     return self.name