[run_wpt_tests.py] Allow passing individual tests or directories

This is required for the MVP builders, which will run a subset of the
WPT tests for Web Payments and Web Identity. It is also useful for local
users of the script, to let them not run the entire WPT test suite.

Bug: 1127055
Change-Id: I8142593a2afd9b2bcd94750765a3f626e5484310
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412813
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Luke Z <lpz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807567}

testing/scripts/run_wpt_tests.py

@@ -37,7 +37,8 @@ class WPTTestAdapter(wpt_common.BaseWptScriptAdapter):
             "--venv=../../",
             "--skip-venv-setup",
             "run",
-            "chrome",
+            "chrome"
+        ] + self.options.test_list + [
             "--binary=../../out/Release/chrome",
             "--binary-arg=--host-resolver-rules="
                 "MAP nonexistent.*.test ~NOTFOUND, MAP *.test 127.0.0.1",
@@ -80,6 +81,10 @@ class WPTTestAdapter(wpt_common.BaseWptScriptAdapter):
         ])
         return rest_args
 
+    def add_extra_arguments(self, parser):
+        parser.add_argument('test_list', nargs="*",
+                            help="List of tests or test directories to run")
+
     def do_pre_test_run_tasks(self):
         # Generate WPT metadata files.
         common.run_command([