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

[FilippoRezzonico]

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.