Flaky PasswordCacheTest failure

[nickva]

The test seem to fail up to 3 or 4 times per for full CI run.

[2024-02-10T04:45:52.525Z] 
[2024-02-10T04:45:52.525Z] PasswordCacheTest [test/elixir/test/password_cache_test.exs]
[2024-02-10T04:45:59.003Z]   * test password cache [L#7]
  * test password cache (5683.2ms) [L#7]
[2024-02-10T04:45:59.003Z] 
[2024-02-10T04:45:59.003Z]   1) test password cache (PasswordCacheTest)
[2024-02-10T04:45:59.003Z]      test/elixir/test/password_cache_test.exs:7
[2024-02-10T04:45:59.003Z]      Slow hash is expected for user1 during login
[2024-02-10T04:45:59.003Z]      code: run_on_modified_server(server_config, fn -> test_fun(db_name) end)
[2024-02-10T04:45:59.003Z]      stacktrace:
[2024-02-10T04:45:59.003Z]        test/elixir/test/password_cache_test.exs:78: PasswordCacheTest.assert_cache/4
[2024-02-10T04:45:59.003Z]        test/elixir/test/password_cache_test.exs:135: PasswordCacheTest.test_fun/1
[2024-02-10T04:45:59.003Z]        (couchdbtest 0.1.0) test/elixir/lib/couch/dbtest.ex:508: Couch.DBTest.run_on_modified_server/2
[2024-02-10T04:45:59.003Z]        test/elixir/test/password_cache_test.exs:23: (test)

Increasing the timeout to 5500 didn't seem to do it. #4980 Maybe it needs to be increased even higher?

If it's impossible to make it stable, we could try putting in in the chttpd suite so we could meck wait to synchronize and wait for internal calls...

[big-r81]

Fixed by #4982.