generic/624: test multiple Merkle tree block sizes

generic/624: test multiple Merkle tree block sizes

Instead of only testing 4K Merkle tree blocks, test FSV_BLOCK_SIZE, and
also other sizes if they happen to be supported.  This allows this test
to run in cases where it couldn't before and improves test coverage in
cases where it did run before.

This required reworking the test to compute the expected digests using
the 'fsverity digest' command, instead of relying on .out file
comparisons.  (There isn't much downside to this, since comparison to
known-good file digests already happens in generic/575.  So if both the
kernel and fsverity-utils were to be broken in the same way, generic/575
would detect it.  generic/624 serves a different purpose.)

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>