[web_tests][resultdb] set testLocation when report a result to rdb-sink

Chromium-deriver is a tool that parses the isolate output of
a test run, and uploads the test results to ResultDB.

Chromium-deriver is currently being used in chromium recipes to upload
the test results of Swarming Tasks to ResultDB, and it has a hard-coded
path for blink_web_tests.
: https://source.chromium.org/chromium/infra/infra/+/master:go/src/go.chromium.org/luci/resultdb/internal/services/deriver/swarming.go;l=190-191
: https://source.chromium.org/chromium/infra/infra/+/master:go/src/go.chromium.org/luci/resultdb/internal/services/deriver/formats/json.go;l=357-359

This CL is to copy the same logic to test_result_sink.py so that
run_web_tests.py with `rdb stream` will generate and upload the same
test results that chromum-deriver uploads for blink_web_tests.

R=robertma@chromium.org
CC=chanli@chromium.org,iannucci@chromium.org,nodir@chromium.org

Bug: 1108016
Change-Id: Ia31bf3435ca9fab70359c64d8485dcd2397c1c07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2425750Reviewed-by: Chan Li <chanli@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Scott Lee <ddoman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810326}

third_party/blink/tools/blinkpy/web_tests/controllers/test_result_sink.py

@@ -168,5 +168,13 @@ class TestResultSink(object):
             # 'startTime': result.start_time
             'tags': self._tags(result),
             'testId': result.test_name,
+
+            # testLocation is where the test is defined. It is used to find
+            # the associated component/team/os information in flakiness and
+            # disabled-test dashboards.
+            'testLocation': {
+                'fileName':
+                '//third_party/blink/web_tests/' + result.test_name,
+            },
         }
         self._send({'testResults': [r]})

third_party/blink/tools/blinkpy/web_tests/controllers/test_result_sink_unittest.py

@@ -84,6 +84,16 @@ class TestResultSinkMessage(TestResultSinkTestBase):
         self.assertEqual(sent_data['status'], 'CRASH')
         self.assertEqual(sent_data['duration'], '123.456s')
 
+    def test_test_location(self):
+        tr = test_results.TestResult('')
+        prefix = '//third_party/blink/web_tests/'
+        sink = lambda tr: self.sink(True, tr)['testLocation']['fileName']
+
+        tr.test_name = "test-name"
+        self.assertEqual(sink(tr), prefix + 'test-name')
+        tr.test_name = "///test-name"
+        self.assertEqual(sink(tr), prefix + '///test-name')
+
     def test_device_failure(self):
         tr = test_results.TestResult(test_name='test-name')
         tr.type = ResultType.Failure