LLVM14 test suite not run on llvmdev pacakage build

[apmasell]

Spawned from #830 The llvm-lit test suite isn't run for LLVM 14 as the tool has changed. There seems to be multiple issues:

• the LLVM instructions for running the test suite no longer match those in the build file and involve other repositories
• it is unclear if llvm-check-unit is a sufficient test and this target isn't generated by cmake on Windows

The unit test files appear to be in llvm/tests. Worth noting that the LLVM-provided Debian packages do not include llvm-lit, so it is unclear if this is a preferred way to run tests.