test-doc-portal: Skip all tests if FUSE isn't supported
test-doc-portal: Skip all tests if FUSE isn't supported

Distribution autobuilders sometimes use a hardened OS configuration that
does not allow module loading or FUSE; in particular, this is the case
in Debian. In this environment, global setup does not complete,
@documents is NULL and this test would segfault. Skip the test and rely
on installed-tests to cover it instead.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@debian.org>

matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal