Certificates that are used in tests will expire in the far future

In repo `tests/fixture/keys` there are certificates (`client.crt`, `server.crt`, `ca.crt`) that will expire on 3 September 3061.
So far it's not a problem, but it could be useful to keep track of this issue.
A possible solution is to generate new certificates every time tests are run or update this certificates before their expire date.

Start in tests/fixture/keys with client.crt, server.crt, and ca.crt, then inspect the tests that consume these certificates. Decide whether the project should regenerate them during test runs or replace them before expiry; done means the chosen approach is implemented and the certificate-based tests continue to pass.

Written by the indexing model from the issue text.