* migrate runtime safety tests to the new test harness
- this required adding compare output / execution support for stage1
to the test harness.
* rename `zig build test-stage2` to `zig build test-cases` since it now
does quite a bit of stage1 testing actually. I named it this way
since the main directory in the source tree associated with these
tests is "test/cases/".
* add some documentation for the test manifest format.
