Some test runners (e.g. transactions, CSFLE) already used a dedicated client. Applying this to all legacy test runners will also allow us to ensure that a separate client is used for internal operations (e.g. assertCollectionExists, assertIndexExists). Combined with per-client event subscribers (PHPC-1666) we can also ensure that observed commands only pertain to actual test operations.

Applying this to assertIndexExists will also address some test failures observed in new CSFLE spec tests (PHPLIB-885) due to CDRIVER-4401. The previous behavior, where assertIndexExists happened to run using the client under test with encryption enabled, was crucial to uncovering CDRIVER-4401, but there is ultimately no reason to continue using the client under test for internal test runner operations.