Missing test coverage for c3/optimizers/c2.py

[lazyoracle]

Is your feature request related to a problem? Please describe.
We don't have good test coverage for c3/optimizers/c2.py. Currently it is at 33%

Describe the solution you'd like
Add tests to check the full C2 Calibration workflow

Additional context
A mock experiment must be provided to mimic the behaviour of hardware calibration. This could be something very rudimentary since we already have a more sophisticated workflow in the examples/Simulated_calibration.ipynb that is also checked as part of our CI.