Increase coverage

[wvangeit]

Now that the coverage reporting is fixed we can see that the coverage has regressed over time. @AurelienJaquier @DrTaDa @anilbey, can you help getting the coverage closer to 100% again?
I guess it's mostly deapext/CMA that needs some work.

[wvangeit]

Btw, i don't think there is a functional test for CMA yet, it would be good to have one (i.e. run a 'very' small optimization with CMA and compare results)

[anilbey]

The built-in unittest.mock library can be handy for the parts of code that are harder to test.
https://docs.python.org/3/library/unittest.mock.html#module-unittest.mock

[DrTaDa]

Partially full-filled with #402 and #403