mach: Allow filtering tests in mach test-devtools (#38514)

`mach test-devtools` uses
[unittest](https://docs.python.org/3/library/unittest.html) without
[unittest.main()](https://docs.python.org/3/library/unittest.html#unittest.main),
so we can parse arguments and set up the test runner how we want, but
this means there’s currently no way to filter tests on the command line.

this patch plumbs the positional arguments (if any) into the
TestLoader’s
[testNamePatterns](https://docs.python.org/3/library/unittest.html#unittest.TestLoader.testNamePatterns),
with extra logic to treat any pattern `pattern` not containing `*` like
`*pattern*`. this is the same behaviour as unittest.main()
[`-k`](https://docs.python.org/3/library/unittest.html#cmdoption-unittest-k):

```
$ ./mach test-devtools domparser responsexml
Running devtools tests...
Running 2 tests:
- test_source_content_inline_script_with_domparser (servo.devtools_tests.DevtoolsTests.test_source_content_inline_script_with_domparser)
- test_source_content_inline_script_with_responsexml (servo.devtools_tests.DevtoolsTests.test_source_content_inline_script_with_responsexml)

test_source_content_inline_script_with_domparser (servo.devtools_tests.DevtoolsTests.test_source_content_inline_script_with_domparser) ... ok
test_source_content_inline_script_with_responsexml (servo.devtools_tests.DevtoolsTests.test_source_content_inline_script_with_responsexml) ... ok

----------------------------------------------------------------------
Ran 2 tests in 4.055s

OK
```

Testing: not really worth automated testing, but tested manually above
Fixes: part of #36325

---------

Signed-off-by: Delan Azabani <dazabani@igalia.com>

python/servo/devtools_tests.py

@@ -317,9 +317,23 @@ class DevtoolsTests(unittest.IsolatedAsyncioTestCase):
         return os.path.join(DevtoolsTests.script_path, os.path.join("devtools_tests", path))
 
 
-def run_tests(script_path, build_type: BuildType):
+def run_tests(script_path, build_type: BuildType, test_names: list[str]):
     DevtoolsTests.script_path = script_path
     DevtoolsTests.build_type = build_type
     verbosity = 1 if logging.getLogger().level >= logging.WARN else 2
-    suite = unittest.TestLoader().loadTestsFromTestCase(DevtoolsTests)
+    loader = unittest.TestLoader()
+    if test_names:
+        patterns = []
+        # unittest.main() `-k` treats any `pattern` not containing `*` like `*pattern*`
+        for pattern in test_names:
+            if "*" in pattern:
+                patterns.append(pattern)
+            else:
+                patterns.append(f"*{pattern}*")
+        loader.testNamePatterns = patterns
+    suite = loader.loadTestsFromTestCase(DevtoolsTests)
+    print(f"Running {suite.countTestCases()} tests:")
+    for test in suite:
+        print(f"- {test}")
+    print()
     return unittest.TextTestRunner(verbosity=verbosity).run(suite).wasSuccessful()

python/servo/testing_commands.py

@@ -355,10 +355,11 @@ class MachCommands(CommandBase):
         return 0 if passed else 1
 
     @Command("test-devtools", description="Run tests for devtools.", category="testing")
+    @CommandArgument("test_names", nargs=argparse.REMAINDER, help="Only run tests that match these patterns")
     @CommandBase.common_command_arguments(build_type=True)
-    def test_devtools(self, build_type: BuildType, **kwargs) -> int:
+    def test_devtools(self, build_type: BuildType, test_names: list[str], **kwargs) -> int:
         print("Running devtools tests...")
-        passed = servo.devtools_tests.run_tests(SCRIPT_PATH, build_type)
+        passed = servo.devtools_tests.run_tests(SCRIPT_PATH, build_type, test_names)
         return 0 if passed else 1
 
     @Command(