fstests: capture some failures to seqres.full

fstests: capture some failures to seqres.full

Whilst trying to debug test failures, I found a few places where
errors needed to be redirected to $seqres.full rather than
/dev/null. This is a collection of all the conversions that haven't
been captured by some other bug fix patch.

Note that calling _check_filesystems() after removing the
require_test/scratch files means it is a no-op, so I removed that
call at the same time as capturing unmount failures after the test
has failed.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>