GODRIVER-2068 Use monitor.TestPoolMonitor to replace most connection pool monitors in tests.

[matthewdale]

GODRIVER-2068

There are various test types that are used as an *event.PoolMonitor and all work slightly differently with different levels of isolation between tests. The existing testPoolMonitor in the mongo/integration package tests can satisfy the use cases of all pool monitors used in tests and is intended to be instantiated per-test to prevent interaction between tests. Move the existing testPoolMonitor into a new test utility package internal/testutil/monitor and use it in most tests that require a connection pool monitor.

Making the new test pool monitor available to all tests also allows us to improve the flaky TestServerConnectionTimeout test by replacing the test-specific pool monitor with a monitor.TestPoolMonitor.

Changes:

• Remove the various test types that are used as an *event.PoolMonitor and replace them with the new monitor.TestPoolMonitor type.
• Remove unused code from the internal/testutil package.

[prestonvasquez]

LGTM!

[benjirewis]

Awesome! This is great cleanup and putting TestPoolMonitor in its own, new package seems best. Great work 🧑‍🔧