Add --conitinue flag to test runner. NFC (#20845)

When combined with `--failfast` this allows you to continue where you
left off.

Author: sbc100

test/common.py

@@ -77,6 +77,7 @@
 NON_ZERO = -1
 
 TEST_ROOT = path_from_root('test')
+LAST_TEST = path_from_root('out/last_test.txt')
 
 WEBIDL_BINDER = shared.bat_suffix(path_from_root('tools/webidl_binder'))
 
@@ -902,6 +903,7 @@ def setUp(self):
       else:
         print('Creating new test output directory')
         ensure_dir(self.working_dir)
+      utils.write_file(LAST_TEST, self.id() + '\n')
     os.chdir(self.working_dir)
 
   def runningInParallel(self):

test/runner.py

@@ -382,6 +382,9 @@ def parse_args(args):
   parser.add_argument('tests', nargs='*')
   parser.add_argument('--failfast', action='store_const', const=True, default=False)
   parser.add_argument('--start-at', metavar='NAME', help='Skip all tests up until <NAME>')
+  parser.add_argument('--continue', dest='_continue', action='store_true',
+                      help='Resume from the last run test.'
+                           'Useful when combined with --failfast')
   parser.add_argument('--force64', action='store_const', const=True, default=None)
   parser.add_argument('--crossplatform-only', action='store_true')
   return parser.parse_args()
@@ -465,6 +468,11 @@ def prepend_default(arg):
     tests = tests_with_expanded_wildcards(tests, all_tests)
     tests = skip_requested_tests(tests, modules)
     tests = args_for_random_tests(tests, modules)
+
+  if not options.start_at and options._continue:
+    if os.path.exists(common.LAST_TEST):
+      options.start_at = utils.read_file(common.LAST_TEST).strip()
+
   suites, unmatched_tests = load_test_suites(tests, modules, options.start_at)
   if unmatched_tests:
     print('ERROR: could not find the following tests: ' + ' '.join(unmatched_tests))