tests: fuse: use stat instead of relying on "ls -l" output

tests: fuse: use stat instead of relying on "ls -l" output

On my Fedora 31 system, this currently fails because the test
expects the file mode bits to be printed as "-rw-r--r--" but
I get "-rw-r--r--." instead, perhaps due to selinux.

Fix this to use stat --format instead to extract the bits we
want to check here (user, group, mode, timestamp).

While at it, I noticed that this test has some confusion with
timezones and only worked because 'ls -l' doesn't show the
time in this case since the file is so old. Fix that by making
_everything_ here use UTC, instead of just some parts.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>