Create better API for WebDriver Bidi Tests

[k7z45]

This is a follow up issue to 27195. There were some remaining conversations but we decided to merge what we had to get things unblocked.

Summary:

1. Creating a bidi_session fixture instead of existing bidi marker to reduce user test code setup for the marker. [1][2]
2. Re-use global current_session instead of creating another global bidi session. [4]
3. [3]

[1] Instead of a marker, which would mean that we have to add it before each and every test method, having a dedicated bidi_session fixture would cause way lesser code to write:

async def test_log_events(bidi_session, url):
    bidi_session.url = url("/tests/bidi/support/log_events.html")
    ...

That way we can also factor out all the bidi specific code from the session fixture itself, and have a clear separation for the BiDi extension.

What do you think?

Originally posted by @whimboo in #27195 (comment)
[2] With the dedicated session approach this can also become part of thebidi_session fixture.

Originally posted by @whimboo in #27195 (comment)
[3] We might need a similar new_session fixture for BiDi so that you can create multiple new sessions with different capabilities within a single test.

Originally posted by @whimboo in #27195 (comment)
[4] I'm worried about having more than one global session. I think it would make sense to have a bidi_session fixture but for it to actually share the same session as the _current_session; if the bidi status changes we'd need to restart the session just like we do with the capabilities.

Originally posted by @jgraham in #27195 (comment)

[whimboo]

That sounds good. @k7z45, will you be able to work on that? Thanks!

[k7z45]

Sure, thanks!

[whimboo]

I think that we can close this issue given that the bidi_session fixture has been implemented quite some time ago. For anything else we can file a follow-up issue if needed.