xfstests: skip dump tests if dump utils not found

xfstests: skip dump tests if dump utils not found

I was running xfstests on a NAS box which had no xfsdump,
and any tests which required xfsdump were failing in not-so-nice
ways:

+ xfsdump not found
038      [failed, exit status 1] - output mismatch (see 038.out.bad)

I think using _notrun is a lot better:

038      [not run] xfsdump not found

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Josef 'Jeff' Sipek" <jeffpc@josefsipek.net>
Reviewed-by: Felix Blyakher <felixb@sgi.com>