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

UltraBlame original commit: e4190bcbd4ef1b4dc8bb21c265c667cd007326d1

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

@@ -344,8 +344,8 @@
 [
 "
 -
--
-strict
+W
+error
 "
 
 "