Update Catch2

[otbrown]

QuEST is currently pinned to Catch2 v2.13. The latest release is v3.7.1 so we should at least consider upgrading. I don't particularly want to do this for the v4 release, but it could be in v4.1.

Note that Catch2 v3 is not header-only like v3, so testing won't be self-contained within this repo any longer, so will need approval from @TysonRayJones!

[TysonRayJones]

Sounds good to me! The old single-header version allowed one to clone QuEST and run the unit tests with "no dependencies", i.e. no user preparation of the Catch2 library (which made for a great live-coding demo). Can we somehow maintain this when using v3+ Catch2? Would we have to submodule Catch2 into the QuEST repo?

(Note it's a minor chore to suppress duplicated Catch2 logging when running distributed tests.)

[TysonRayJones]

Wew I've since learned this can be done with CMake's FetchContent, like the below demo (from the Catch2 doc):

Include(FetchContent)

FetchContent_Declare(
  Catch2
  GIT_REPOSITORY https://github.com/catchorg/Catch2.git
  GIT_TAG        v3.4.0 # or a later release
)

FetchContent_MakeAvailable(Catch2)

add_executable(tests test.cpp)
target_link_libraries(tests PRIVATE Catch2::Catch2WithMain)

Alas I won't be able to effect a patch/diff to avoid duplicated output when running distributed - I'll instead need to write my own reporter 😢 I'll procrastinate that until the tests are all up and working 🎉