overlay: Add test coverage for fs-verity support

overlay: Add test coverage for fs-verity support

This tests that the right xattrs are set during copy-up, and
that we properly fail on missing of erronous fs-verity digests
when validating.

We also ensure that verity=require fails if a metacopy has not
fs-verity, and doesn't do a meta-coopy-up if the base file lacks
verity.

Signed-off-by: Alexander Larsson <alexl@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>