test: add concurrency test

[taras-zak]

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Description

• Redis subscriptions have at-most-once semantic and publishing event for topic that not have subscribers will lead to missing this event. TestPublishBeforeSubscribe cover this behaviour, we can see that "before subscribe" event (even "publish immediately" sometimes) not reached subscriber:

=== RUN   TestPublishBeforeSubscribe
2025/01/10 13:04:38 after subscribe
--- PASS: TestPublishBeforeSubscribe (5.01s)
PASS

=== RUN   TestPublishBeforeSubscribe
2025/01/10 13:07:20 publish immediately
2025/01/10 13:07:20 after subscribe
--- PASS: TestPublishBeforeSubscribe (5.01s)
PASS

• So cache-lib have bug, when concurrent requests might stuck waiting notification for event. Here is results of the TestConcurrentRequestsForSameKey runs, we can see that it not stable:

=== RUN   TestConcurrentRequestsForSameKey
2025/01/10 12:58:24 miss from 1
2025/01/10 12:58:25 first request finished &{true} <nil>
2025/01/10 12:58:30 second request finished <nil> error reading from pub/sub
    cache_test.go:250: 
        	Error Trace:	/Users/taras/projects/cache-lib-go/cache_test.go:250
        	Error:      	Received unexpected error:
        	            	error reading from pub/sub
        	Test:       	TestConcurrentRequestsForSameKey
--- FAIL: TestConcurrentRequestsForSameKey (5.11s)

FAIL

Another run might pass

=== RUN   TestConcurrentRequestsForSameKey
2025/01/10 13:01:58 miss from 1
2025/01/10 13:01:58 first request finished &{true} <nil>
2025/01/10 13:01:58 second request finished &{true} <nil>
--- PASS: TestConcurrentRequestsForSameKey (0.11s)
PASS

• fnx-api-gateway has ~100 errors per hour during peak time because of this bug - grafana

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This change is