build: automatically allocate an X server number in the unit tests.

build/lol-build

@@ -265,7 +265,7 @@ check()
             # exists, use it to run the test suite.
             if test "x${MSYSTEM}${DISPLAY}" = x \
                && xvfb-run --help 2>&1 >/dev/null; then
-                xvfb-run make check
+                xvfb-run -a make check
             else
                 make check
             fi