[php-src] PR #24999: Fix flaky FPM test for bug #77023

Pull Request: https://github.com/php/php-src/pull/24999 Author: bukka The test sent two parallel requests to a pool with a single child and expected the dynamic process manager to spawn a second child. That only happens when the idle server maintenance heartbeat (1s) fires while the child is busy, but the scripts kept it busy for just 300ms each, so the spawn was a matter of luck and the total processes check failed on slow machines. Use a single request that blocks the child for longer than the heartbeat so the spare child is always spawned, and rely on the fact that the blocked child is the oldest idle one and is therefore quit first. One blocked child is enough to verify the SIGKILL fallback. Replace the fixed sleep with log timeouts and poll the status until the spare child is quit.