xfstests: add background noise to test 276 (btrfs backref resolving)

xfstests: add background noise to test 276 (btrfs backref resolving)

The backref resolver test for btrfs was run on a static file system so far.
Resolving backrefs on a busy file system is what happens in reality and that
is what should be checked by this test.

I added a parameter such that the script can easily be changed to the former
behavior for development purposes.

Furthermore, this increases two constants which make the test simply cycle a
few seconds longer, increasing the chance to hit on something suspicious in
case we broke something.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>