In Emscripten builds, remove comparisons using `fs::last_write_time`
because they cause a compilation error within Catch2. Add sleep calls
to lighten the CPU load when launching successive game runs.
Also added a script for upgrading to the latest version of Catch2 and
upgraded Catch2 to v3.7.1.
- Added a simple test program that loads Catch2 and opens and closes a game
- Added build targets for Linux and Emscripten for the test program
- Game class is no longer pure abstract, so it can be instantiated directly
- Changed glcontext to a shared_ptr so it will be deleted automatically
