Skip some tests if we can't use FUSE

On some systems we can't make use of FUSE, especially on the same
locked-down systems where distribution packages are typically built.
For example, official Debian autobuilders (buildds) are configured to
disallow module loading after boot has finished as a form of security
hardening, some build chroots don't have a valid /etc/mtab, and Docker
containers give us uid 0 but not CAP_SYS_ADMIN.

These checks are taken from libostree.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #2824
Approved by: matthiasclasen
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