xfs: test mkfs.xfs config files

Simple tests of the upcoming mkfs.xfs config file feature.  First we
have some simple tests of properly formatted config files, then
improperly formatted config files, and finally we try to spot
conflicts between config file options and the cli.

[dchinner: updated for new libinih-based implementation.]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>