tests: use our own XDG_RUNTIME_DIR for tests

Use $XDG_RUNTIME_DIR/wayland-tests for tests. This way we won't be
messing XDG_RUNTIME_DIR and it also fixes a bug, when socket-test
failed when another compositor was running.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>