PSA: --ignore-testharness-expected-txt for run_web_tests.py

[Xianzhu Wang]

Hi,

We have more than 2000 WPT tests that actually partially fail but pass under run_web_tests.py because they have -expected.txt containing failing results. Now run_web_tests.py has a new option --ignore-testharness-expected-text to show these failures by ignoring these -expected.txt. For now this is for local usage (i.e. not on bots), e.g. for an owner of a WPT subdirectory to know which tests are failing and need fixing.

In the future we will have a better mechanism to suppress testharness test failures with better bug tracking.

The flag also applies to non-WPT testharness tests.

Xianzhu