xfs: test verity with continuous data and merkle tree

fs-verity metadata is stored at the next 64k offset after EOF. This
means for file of size 64k the merkle tree would be stored exactly after
EOF, therefore in the next block following last data block. This means
that these extents could be merged. Test that verity still works in this
geometry.

Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>