Automated tests

[sosthene-nitrokey]

• [x] Basic test for signature against a demo instance of the nethsm
• [x] Test with a firewall killing a connection (timeout respected)
• [x] Test with a firewall and another instance avaliable (requests function)
• [ ] Tests with another instance (round-robin is used)
  • [x] Test that all instances are used at least once
  • [ ] Test that instances are used with the right rate (this is harder since we need to somehow keep track of connections to the proxies).

Not sure how this will all be made to fit with the CI, especially the firewall test since that will require iptables access.

To make testing easier, this PR also makes the C_Initialize re-initialize the config, allowing to dynamically configure the module.

• [x] Make C_Finalize also discard the previous config