tests: don't compare btrfs mount options

tests: don't compare btrfs mount options

We don't control mount options in the mountinfo file and the options
depend on kernel version. It does not make sense to hardcode the
options in the expected outputs.

Note that libmount should be portable enough to be able to work with
all kernel versions. It seems better to check libmount functionality
than kernel mount options formatting.

Signed-off-by: Karel Zak <kzak@redhat.com>