tests: Pass --backend so the test suite runs with the right modules

Without this we try to load the installed backends, which is nasty for
regular runs, and just doesn't work for make distcheck, which sets
prefix to $PWD/_inst.  This makes sure we load the right backend
and make distcheck pass.  Other modules (xwayland, shells etc) just don't
get loaded for distcheck and for make check we still try to load the
installed modules.

tests/weston-tests-env

@@ -10,14 +10,24 @@ OUTLOG="$LOGDIR/$1-log.txt"
 
 rm -f "$SERVERLOG"
 
+if test x$WAYLAND_DISPLAY != x; then
+	BACKEND=$abs_builddir/../src/.libs/wayland-backend.so
+elif test x$DISPLAY != x; then
+	BACKEND=$abs_builddir/../src/.libs/x11-backend.so
+else
+	BACKEND=$abs_builddir/../src/.libs/wayland-backend.so
+fi
+
 case $1 in
 	*.la|*.so)
-		$WESTON --modules=$abs_builddir/.libs/${1/.la/.so} \
+		$WESTON --backend=$BACKEND \
+			--modules=$abs_builddir/.libs/${1/.la/.so} \
 			--log="$SERVERLOG" \
 			&> "$OUTLOG"
 		;;
 	*)
 		WESTON_TEST_CLIENT_PATH=$abs_builddir/$1 $WESTON \
+			--backend=$BACKEND \
 			--log="$SERVERLOG" \
 			--modules=$abs_builddir/.libs/weston-test.so \
 			&> "$OUTLOG"