common/encrypt: verify the key identifiers

common/encrypt: verify the key identifiers

As part of all the ciphertext verification tests, verify that the
filesystem correctly computed the key identifier from the key the test
generated.  This uses fscrypt-crypt-util to compute the key identifier.

Previously this was only being tested indirectly, via the tests that
happen to use the hardcoded $TEST_RAW_KEY and $TEST_KEY_IDENTIFIER.
The new check provides better coverage.

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