Add mocking

[malachib]

As per https://github.com/catchorg/Catch2/issues/1659 we have some interesting options:

• https://github.com/rollbear/trompeloeil
• https://github.com/matepek/catch2-with-gmock
• https://github.com/sinojelly/catch2-with-mockcpp

One gets the impression trompeloeil is the best choice. It appears to have direct Catch2 integration