t/lib-git-p4: silence output when killing p4d and its watchdog

t/lib-git-p4: silence output when killing p4d and its watchdog

When stopping the p4d watchdog process via "kill -9", the shell may
print a job-control notification like:

  ./test-lib.sh: line 1269: 57960 Killed: 9               while true; do
      if test $nr_tries_left -eq 0; then
          kill -9 $p4d_pid; exit 1;
      fi; sleep 1; nr_tries_left=$(($nr_tries_left - 1));
  done 2> /dev/null 4>&2  (wd: ~)

This message is printed asynchronously by the shell when it reaps the
process. While harmless right now, this will cause breakage once we
enable strict parsing of the TAP protocol in a subsequent commit.

Fix this by using `wait` so that we can synchronously reap the watchdog
process and swallow the diagnostic.

While at it, deduplicate the logic we have in `stop_p4d_and_watchdog ()`
and `stop_and_cleanup_p4d ()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>