Bug 1470530 - [wptrunner] Correct the warning to error option for pyt…

…est. r=ato

Currently the "--strict" option is used to turn warnings into
errors. But even the help content says that, it is not true.
Instead this option is only used for markers.

To actually turn warnings into errors the "-W error" option
would have to be used.

More details see:
pytest-dev/pytest#2444

MozReview-Commit-ID: AQMwZlwC46w

testing/web-platform/tests/tools/wptrunner/wptrunner/executors/pytestrunner/runner.py

@@ -51,7 +51,7 @@ def run(path, server_config, session_config, timeout=0):
 
     with TemporaryDirectory() as cache:
         try:
-            pytest.main(["--strict",  # turn warnings into errors
+            pytest.main(["-W error",  # turn warnings into errors
                          "-vv",  # show each individual subtest and full failure logs
                          "--capture", "no",  # enable stdout/stderr from tests
                          "--basetemp", cache,  # temporary directory