MDEV-25857 MTR should report at least last test that was executed in case of shutdown and not-completed

* return a success/failure value from mysqld_start()
  and don't error out / exit in mysqld_start(), the caller will do
* pass the correct $mysqld object into check_expected_crash_and_restart()
  instead of searching for it inside. Search in the caller
* so that when a failed restart changes $mysqld->{proc}, mtr would
  still detect it as a failed mysqld (by updating $proc to match)

also: log the server command line into the server error log