[test-runner] Allow to filter out 404 network requests warnings

[web-padawan]

We have a test that submits a form in <iframe> and it logs a warning, which is by design:

packages/vaadin-login/test/login-submit.test.js:

 🚧 404 network requests:
    - login-action?username=username&***

It would be nice to be able to filter out these warnings, same as we can do it for console warnings.