btrfs: add fstest for 8k write spanning two stripes on raid-stripe-tree

btrfs: add fstest for 8k write spanning two stripes on raid-stripe-tree

Add a test-case writing 8k to a raid-stripe-tree formatted filesystem with
one stripe pre-filled to 60k so the 8k are split into a 4k write finishing
stripe 1 and a 4k write starting the next stripe.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
[Fixed the test statement and trailing white space in the .out file.]
Signed-off-by: Zorro Lang <zlang@kernel.org>