xfstests: delay before dumps for incremental tests

xfstests: delay before dumps for incremental tests

To get reproducible results when testing incremental backups, files
contained in a backup should not have the same timestamp (at second
resolution) as the backup itself. If they do, those files will also
be included in the incremental backup, and this will likely cause
issues in the expected output of the test. This patch adds a sleep
to test 024 prior to doing the level 0 backup. The other incremental
tests already include similar delays.

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