tests: Use unique XDG_RUNTIME_DIR

Rather than using a hardcoded 'wayland-tests' directory under
the existing XDG_RUNTIME_DIR to use as the new runtime dir, use mkdtemp
to guarantee uniqueness. This fixes make -jN check, as well as just
happening to run 'make check' twice from the same session.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>