Improve test coverage for retryable handshake errors (reactive streams)

The spec tests for JAVA-4798 pass for the sync driver, but fail for the reactive streams driver due to timing-related issues that cause assertion failures on the number of delivered events. Tried using SyncMongoClient#enableSleepAfterCursorOpen but it didn't solve the problem: depending on how long the sleep was it either generated more than or less than the expected number of connection-checkout-started events.

JAVA-4798 tests have been synced and that ticket is closed, but the new tests are skipped in com.mongodb.reactivestreams.client.unified.UnifiedRetryableReadsTest.

This ticket is intended to track work to enable those tests, though it's unclear how it will be done.