Fix TestLimitConcurrency_queueWaitTimeRealTimeout flake

For #5622 (closed)

Similar to the flake in #5601 (closed), this flake is caused by the low timeout of the concurrency limiter.

This test setups two requests:

• The first one is eligible. It enters the handler and blocks the queue.
• The second request is blocked until timeout.

Both of them share this timeout. Internally, the limiter creates a context deadline to reject timed-out requests. If it's set too low, there's a tiny possibility that the context reaches the deadline when the limiter checks the request. Thus, setting a reasonable timeout here and adding some retry attempts below make the test stable.

Although this test can use a mock function, the whole purpose of the test is to verify the code works with a real timeout. Other tests in the same file use the timeout mock extensively already. So, it would be nicer if we can adjust the timeout.