Apps separation #2: Add test app

[ggVGc]

Two apps can now be built and run. Example:

• Go to brains2
• Run nix-shell
• Run ./scripts/build.sh apps/test_app
• Run ./scripts/send_firmware.sh apps/test_app
• Same work as expected for apps/duo
• BOARDS define should be respected as before

Notes

• Each app has its own minimal CMakeLists.txt which is the entry point for the build and where the executable name and sources are defined, but the main work is done by including the build_app.cmake file.
• The duo app is not yet ported to the App interface. I think we should hold off on that until we've used this for something and seen that it works alright.
• Some manual care should be taken to not call MIDI::init() from something using the App interface, since then the sysex handler will be reinitialized and won't respond to messages to enter bootloader. This shouldn't be an issue for our use-cases.

[ggVGc]

Remade this PR on main so it merges cleanly, and both DUO and test app builds.