tests: use the headless backend to run the test suite

Other backends can be used by passing BACKEND=some-backend.so, e.g.

    $ make check BACKEND=x11-backend.so

Signed-off-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>

tests/weston-tests-env

@@ -17,14 +17,12 @@ OUTLOG="$LOGDIR/$1-log.txt"
 
 rm -f "$SERVERLOG"
 
-if test x$WAYLAND_DISPLAY != x; then
-	BACKEND=$abs_builddir/.libs/wayland-backend.so
-elif test x$DISPLAY != x; then
-	BACKEND=$abs_builddir/.libs/x11-backend.so
-else
-	BACKEND=$abs_builddir/.libs/wayland-backend.so
+if test -z "$BACKEND"; then
+	BACKEND=headless-backend.so
 fi
 
+BACKEND=$abs_builddir/.libs/$BACKEND
+
 case $TESTNAME in
 	*.la|*.so)
 		$WESTON --backend=$BACKEND \