test: fix unit/crypto test flakiness

One of subtests was checking if crypto_decode returns an error
when fails to decode. But due to randomness of the test sometimes
it happened, that initial vector of encrypted data somehow didn't
lead to an error. Decryption was not correct, but only in terms
of result, not in terms of decryption algorithm. -1 was not
returned, and diag was not set.

This patch checks all the cases.

Closes #4306