[php-src] PR #24998: Fix flaky FPM log-bwd-multiple-msgs-stdout-stderr test and remove XFAIL

Pull Request: https://github.com/php/php-src/pull/24998 Author: bukka The end of request flush command is written only to the stderr pipe so a stdout message without a trailing newline is terminated only when the master sees the switch to stderr. If the master is not scheduled between the two requests (slow CI), it reads both stdout chunks at once and logs them as a single "msg 1 - msg 1 - " line which made the test fail. Check the log lines after each request so the master always drains the pipes before the next request is sent. This also verifies the output of the second request which was previously not checked.