This matches master branch. We can look into adding more target coverage
as we switch to stage2. As it stands, this works around having to
duplicate the "Executor" logic to figure out when to not run the tests
due to them being non-native.
* 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.
