xfstests: use value of FSTYP if defined externally

The initial value of FSTYP is unconditionally set to 'xfs' and the
filesystem type is taken from the TEST_DEV. This could lead to confusion
if the device hasn't been formatted yet, eg. an empty image in VM, or
a different test setup took place before.

Now one can specify the desired FSTYP in advance and be safe. If unset,
the fallback to TEST_DEV type continues to work.

Signed-off-by: David Sterba <dsterba@suse.cz>
xfstests: don't suggest to run full diff when DIFF_LENGTH is 0

If DIFF_LENGTH is set to 0, the full output is available and the message
does not make sense.

Signed-off-by: David Sterba <dsterba@suse.cz>