[tools] Print variants in test runner

Print ALL_VARIANTS and VARIANT_ALIASES when passing a wrong
--variants argument to tools/run-tests.py.

Change-Id: I6d4278633dd11990d0ace1c93f544213fbfc156e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2139579
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67039}

tools/testrunner/standard_runner.py

@@ -259,6 +259,8 @@ class StandardTestRunner(base_runner.BaseTestRunner):
     for v in user_variants:
       if v not in ALL_VARIANTS:
         print('Unknown variant: %s' % v)
+        print('    Available variants: %s' % ALL_VARIANTS)
+        print('    Available variant aliases: %s' % VARIANT_ALIASES.keys());
         raise base_runner.TestRunnerError()
     assert False, 'Unreachable'