samples: net: dumb_http_server_mt: Quit if client says so

If the client sends OK or FAIL, then it means that we are run
under a test system and we should stop running and exit so that
the test runner (scripts/net/run-sample-tests.sh) can report
success or failure.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>