fstests: add btrfs fs-verity send/recv test

fstests: add btrfs fs-verity send/recv test

Test btrfs send/recv support for fs-verity. Includes tests for
signatures, salts, and interaction with chmod/caps. The last of those is
to ensure the various features that go in during inode_finalize interact
properly.

This depends on the kernel patch adding support for send:
btrfs: send: add support for fs-verity

And the btrfs-progs patch adding support for recv:
btrfs-progs: receive: add support for fs-verity

Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>