Before this fix, the test thread_cache failed with spurious failures.

The test used:
-- disconnect X
-- connect Y

while assuming that connection Y would reuse connection X slot in the thread cache.

For this to happen, the disconnect X operation must be given enough time to complete,
otherwise connect Y can be executed in the server before X actually finishes.

This fix uses wait conditions to make the test execution more controlled,
and more reproductible.