PYTHON-5696 Fix racy backpressure-network tests

[blink1073]

PYTHON-5696

Changes in this PR

Update backpressure-network sdam tests to avoid race condition with server heartbeats between members of a replica_set. Testing for mongodb/specifications#1880.

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?

[blink1073]

Test failures are unrelated, I need to update the backpressure branch

[blink1073]

The integration and static checks will be fixed by #2635

[aclark4life]

How does this avoid the race condition?

[blink1073]

How does this avoid the race condition?

We wait for the server topology to get into a known state before adding the failCommand, to avoid the case where we haven't connected to all of the servers yet.

[aclark4life]

LGTM