xfstests: refactor cumulative restore tests

xfstests: refactor cumulative restore tests

The cumulative restore tests call _do_restore_file_cum(),
which requires a dump level to be passed in the -l option
in order to determine whether the restore directory needs
to be prepared or not. -l is not a valid xfsrestore option,
so doing things this way prevents tests from passing options
to xfsrestore. It's more straightforward to have the test
call _prepare_restore_dir itself prior to starting a series
of cumulative restores.

Signed-off-by: Bill Kendall <wkendall@sgi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>