tests: allow weston test plugin to keep on running

tests: allow weston test plugin to keep on running

If the environment variable WESTON_TEST_CLIENT_PATH is not set, do not
quit Weston in the test plugin.

This allows one to start Weston with the test plugin manually, and then
run any tests also manually, while observing Weston's behaviour over
time. This is useful for:
- Running a test multiple times and checking if Weston leaks (e.g. with
  Valgrind)
- Running tests manually on a backend that is not x11 or wayland,
  especially the backends that require weston-launch, and therefore
  cannot be used with the 'make check' machinery.

This change should not affect 'make check' behaviour, because there
WESTON_TEST_CLIENT_PATH is always set.

Cc: U. Artie Eoff <ullysses.a.eoff@intel.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>