xfstests: add 267 and 268 for multiple media files

xfstests: add 267 and 268 for multiple media files

This patch adds a couple of tests for xfsdump when multiple media files
are used. 267 tests the case where a file is split across multiple media
files, and 268 tests the case where a file ends on one media file and
the next media file starts on another file. These tests use a small
media file size (xfsdump -d) so that they don't rely on having to hit
end-of-tape.

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