stage2: clean up test harness, implement symbol collision detection (#5708)
* Clean up test harness
* Stage2/Testing: Add convenience wrappers
* Add a `compiles` wrapper case
* fix incremental compilation after error
* exported symbol collision detection
* function redefinition detection for Zig code
* handle missing function names
* Stage2/Testing: Simplify incremental compilation tests
* Stage2/Testing: Update documentation
* Stage2/TestHarness: Improve progress reporting
* Disable test
* Improve Tranform failure output