Write additional external linkage/runtime tests with LIT once it is introduced

[mhasel]

Writing tests which link to external libraries is quite tricky with our current test-setup (see std-lib tests). This necessitates manual testing when making changes to e.g. global variables (#1188).
Additionally, all of our runtime tests are only tested against JIT-compiled binaries. Once we've introduced the LIT testing-framework, we should take advantage of the ease of use and restructure/add additional tests to more thoroughly test runtime behaviour and ensure linked libraries (written in ST and C) behave the way we expect them to.