d/rules, d/with-temp-xdg-runtime-dir: Create temporary XDG_RUNTIME_DIR

debhelper 13.1 creates a temporary XDG_RUNTIME_DIR so we don't have to.
Unfortunately, its absolute path is sufficiently long that the path to
Mutter's Wayland display socket no longer fits in the 108 bytes allowed
by struct sockaddr_un, causing FTBFS when the tests fail. Until this is
fixed, we'll have to create our own XDG_RUNTIME_DIR with a shorter
absolute path.

Wrapping with-temp-xdg-runtime-dir around dh_auto_test doesn't work,
because dh_auto_test will reset XDG_RUNTIME_DIR, so use
meson test --wrapper to wrap with-temp-xdg-runtime-dir around the
individual tests.

Workaround for #961655.