tests: change strategy for `test_ensure_max_concurrency_is_observed`

[bnjbvr]

This test started to fail intermittently, so I rewrote it slightly:

• instead of assuming that sleeping for `delay/2 milliseconds will be sufficient (very flakey in the code cov scenario), wait for 2 seconds
• instead of delaying the responses for delay milliseconds, delay them for 60 seconds

That way, no response should get through the client, and 2 seconds should enough time to spawn all the requests to the mock endpoint, so the test should be more precise.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.23%. Comparing base (7d1bbfa) to head (5979059). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4018 +/- ## ======================================= Coverage 84.23% 84.23% ======================================= Files 266 266 Lines 28349 28349 ======================================= + Hits 23879 23881 +2 + Misses 4470 4468 -2 ```

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