Catch2: Add mocking support

[Viatorus]

Description
If you want to mock an interface in C++, there are two well known frameworks, which are:

• GMock feature-rich but with an old school API and no seamless integration with Catch (GMock throws exceptions on mocking failure)
• FakeIt with a nice API, but sadly, by using some compiler internal vtable modification. It does not support all kinds of mocking (no virtual inheritance, no multiple inheritance, optimization limitation).

I have the strong opinion, that we can create a better API than GMock (by supporting only newer CPP standards) but overcome the limitation of FakeIt.

Additional context
I would love to support this feature with planning and PRs.

Guys, should we give it a try?

[Viatorus]

There is https://github.com/rollbear/trompeloeil.

I think it is a good alternative to all.

[ChooseYourPlan]

So no further development in this?

[matepek]

This is might interesting for someone: https://github.com/matepek/catch2-with-gmock