[test] Make variants flag of numfuzz compatible with test runner

Prepares for: https://crrev.com/c/1015000

NOTRY=true

Bug: chromium:830557
Change-Id: I51c12867d95de19350e0e55aaf097bbb479348dc
Reviewed-on: https://chromium-review.googlesource.com/1015004
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52657}

tools/testrunner/num_fuzzer.py

@@ -81,6 +81,10 @@ class NumFuzzer(base_runner.BaseTestRunner):
     parser.add_option("--combine-min", default=2, type="int",
                       help="Minimum number of tests to combine")
 
+    # Miscellaneous
+    parser.add_option("--variants", default='default',
+                      help="Comma-separated list of testing variants")
+
     return parser
 
 
@@ -97,6 +101,10 @@ class NumFuzzer(base_runner.BaseTestRunner):
                options.min_group_size, options.max_group_size)
         raise base_runner.TestRunnerError()
 
+    if options.variants != 'default':
+      print ('Only default testing variant is supported with numfuzz')
+        raise base_runner.TestRunnerError()
+
     return True
 
   def _get_default_suite_names(self):