backend=auto (now the default if backend is omitted) means to let the compiler
pick whatever backend it wants as the default. This is important for platforms
where we don't yet have a self-hosted backend, such as loongarch64.
Also purge a bunch of redundant target=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.
