generic: test that encryption nonces are unique and random

generic: test that encryption nonces are unique and random

Test that encryption nonces are unique and random, where randomness is
approximated as "incompressible by the xz program".

This gets indirectly tested by generic/399, but there are some gaps.
It's good to test for this directly too.

This test runs and passes on ext4 and f2fs.  It doesn't currently run on
ubifs because _get_encryption_nonce() isn't implemented for ubifs yet.
(At some point I'll probably switch _get_encryption_nonce() to use
FS_IOC_GET_ENCRYPTION_NONCE, which was added in Linux 5.7.  But for now
I'd like to keep the tests using it runnable on older kernels too.)

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>