Use doctest instead of GoogleTest

[2xB]

GoogleTest fails to compile on recent clang versions (see #85 ). GoogleTest currently considers to add a new dependency to the central Google base library "abseil" ( google/googletest#2883 ). Since our fork of GoogleTest is many years out of date and integrating a new version would be a significant effort anyways, it seems reasonable to move to the dependency-less single-header testing library doctest:
https://github.com/doctest/doctest