De-couple test runtime environment from generation environment

[foundrytom]

Following on from this fun, @feltech noted that:

[plan] This conflict only occurs due to tests, so it's a real shame it limits us in our production code.

I think the correct thing to do is isolate the CMake tests somehow. It is already a bit odd the way we build and run a CMake project from within a pytest test case. But that's obviously not a small fix.

Originally posted by @feltech in #31 (comment)

It'd be great if we could decouple the test runtime from the generation runtime, so we don't have this conflict in the first place, as it's not a traitgen conflict per-se.

[elliotcmorris]

This will get rid of the weirdness where pytest launches cmake/ctest, which is weird ... it's just weird okay. And it's hard to work with.