[Discussion] Migration of testing framework from doctest to googletest

[mcbarton]

The following PR in CppInterOp compiler-research/CppInterOp#483 has shown GoogleTest to be a testing framework that works for both native and Emscripten builds (testing the Emscripten builds via node). Therefore switching from Doctest to GoogleTest would be beneficial, as it would allow for automated testing in both environments, and check that no changes to improve native builds, breaks the Emscripten build. It will also allow reviewers to check PRs more easily which claim to enable new features for xeus-cpp-lite, or whether a new feature being added for the native builds also works for the Emscripten build.