MTR Safe Process Hardcodes RLIMIT_NOFILE

The mariadbd instances that are spawned by MTR have their RLIMIT_NOFILE hardcoded to 1024. This makes MTR unusable if used for high concurrency testing (e.g. for benchmarking).

The hardcoding is set in ./mysql-test/lib/My/SafeProcess/safe_process.cc in its main function:

    /*

      mysqld defaults depend on that. make test results stable and independent

      from the environment

    */

    setlimit(RLIMIT_NOFILE, 1024, 1024);

Instead, the safe process executable should use the test's configuration and use its value for open_files_limit.