Update course materials to reflect CMake-based workflow

Once my PR into the exercises repo is merged (https://github.com/ImperialCollegeLondon/unit_testing_Cpp_exercises/pull/1) and we have agreed on the structure of the code etc. we're giving to the students, it might be helpful to update the course materials to include the relevant CMake commands for building (and running?) tests, so that everyone can follow along with the instructions.

Currently the text talks about how to build your test program by invoking gcc directly, which is fine as an explanation of what's going on under the hood, but won't work for students using Visual Studio on Windows, for example. The CMake commands I mention in the README should work for everyone (let me know if they don't!).

Start with the existing course text that explains building tests with gcc and the README in the linked exercises repository, especially its CMake commands. Confirm the agreed exercise structure before editing; done means the course materials explain the CMake build and test workflow in a way that also works for Visual Studio users.

Written by the indexing model from the issue text.