Have a single test to test bidi session with different capabilities

[k7z45]

Posted by @whimboo #27687 (comment)
As mentioned in my reviews for the initial PR (and what @jgraham also referred to earlier in this PR) lets get an issue filed so we can make sure to have a single test for that. It would most likely need the new_session fixture, or maybe a new one for BiDiSession.

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.

Also, by @jgraham
The main part of this looks reasonable. I'm somewhat nervous about all the tests that are basically testing the infrastructure itself and depend on everything running in source order.
#27734 (review)