* `-Dskip-compile-errors` is removed; `-Dskip-stage1` is added.
* Use `std.testing.allocator` instead of a new instance of GPA.
- Fix the memory leaks this revealed.
* Show the file name when it is not parsed correctly such as when the
manifest is missing.
- Better error messages when test files are not parsed correctly.
* Ignore unknown files such as swap files.
* Move logic from declarative file to the test harness implementation.
* Move stage1 tests to stage2 tests where appropriate.
Some cases had to stay behind, either because they required complex case
configuration that we don't support in independent files yet, or because
they have associated comments which we don't want to lose track of.
To make sure I didn't drop any tests in the process, I logged all
obj/test/exe test cases from a run of "zig build test" and compared
before/after this change.
All of the test cases match, with two exceptions:
- "use of comptime-known undefined function value" was deleted, since
it was a duplicate
- "slice sentinel mismatch" was renamed to "vector index out of
bounds", since it was incorrectly named
