Expand test cases for COSE cryptography functions

[pulsastrix]

#13 adds functions to perform cryptographic operations on COSE structures using predefined backends.

While the PR added a large number of test suites (mainly) based on the cose-wg/Examples repository, those test cases do not cover all possible errors that should be handled, and therefore the test suite should be changed accordingly.¹

Footnotes

1. Also, the test coverage does not reach our intended target of 90%. ↩