check: optionally check env var for us to make sure test env is set up
check: optionally check env var for us to make sure test env is set up

If GST_CHECK_TEST_ENVIRONMENT_BEACON is defined, check if the
environment variable it is defined to is set up at the start
of each test.

https://bugzilla.gnome.org//show_bug.cgi?id=747624

centricular/gstreamer,mparis/gstreamer,centricular/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,mparis/gstreamer,mparis/gstreamer,jpxiong/gstreamer,centricular/gstreamer,krichter722/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,mparis/gstreamer