xfstests: add a new test that runs fsstress under ENOSPC conditions

xfstests: add a new test that runs fsstress under ENOSPC conditions

During stress testing we want to cover as much code paths as possible
fsstress is very good for this purpose. But it has expandable nature
(disk usage almost continually grow). So once it goes in no ENOSPC
condition it will be where till the end. But by running 'dd' writers
in parallel we can regularly trigger ENOSPC but only for a limited
periods of time because each time it opens the same file with O_TRUNC.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>