libtest: Skip FUSE if CAP_SYS_ADMIN not in bounding set

libtest: Skip FUSE if CAP_SYS_ADMIN not in bounding set

I'm not sure if something changed or there was a race condition , or
if extraterrestrials intervened to make the tests pass before, but
in the Docker container `/dev/fuse` does exist and since we appear
to be running as uid 0 it'll be writable.

We previously discussed that another requirement for FUSE (and most
setuid binaries) is having `CAP_SYS_ADMIN`, so let's check for that in
the bounding set too.

Closes: #266
Approved by: jlebon