Facilitating automation with browser-specific configuration

[jugglinmike]

In gh-10742 web-platform-tests/results-collection#552, @jgraham wrote:

In general I wish we would put these flags in wptrunner directly rather than having special configuration here.

Since this will expose less authentic behavior, I presume the functionality should be opt-in. Two implementations come to mind:

• Enable all known automation-enabling flags (regardless of the test being executed) via a new command-line option (e.g. wpt run --max-automation firefox)
• Via per-test metadata (e.g. <meta name="automation" content="autoplay,usermedia">)

The latter will be less ergonomic to write and slower to execute, and it likely requires more infrastructural change. Still, it's appealing because it improves the descriptive quality of each test and it allows us to run tests in the most authentic configuration as is possible.

We could also combine the two solutions, requiring per-test metadata but only honoring it in the presence of a new CLI option.

[jugglinmike]

On second thought, this functionality may not be sufficiently distinct from the feature being discussed at gh-10452. I've asked folks there to help decide.