Flaky Test_Concurrency in the query backend client package

[kolesnikovae]

As we use the real transport in tests, if fails from time to time:

=== FAIL: pkg/experiment/query_backend/client Test_Concurrency (0.01s)
    client_test.go:108: 
        	Error Trace:	/home/runner/work/pyroscope/pyroscope/pkg/experiment/query_backend/client/client_test.go:108
        	Error:      	Received unexpected error:
        	            	listen tcp 127.0.0.1:36467: bind: address already in use
        	Test:       	Test_Concurrency

https://github.com/grafana/pyroscope/actions/runs/13364982979/job/37321769113?pr=3885

The failure reasons vary (timouts, EOF, etc.) but the solution is the same – use in-memory transport for tests that involve networking.