PYTHON-5530 Reduce usage of legacy test runner

[blink1073]

PYTHON-5530

Changes in this PR

Reduce usage of legacy test runner by only running the unified CSFLE tests. For the transaction tests it was already a no-op.
We cannot remove the test runner entirely because command_monitoring and server_selection still require a custom runner.

Test Plan

Standard tests.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[sleepyStick]

LGTM!