- Document `allow-non-lint-warnings.rs`
- Move `allow-non-lint-warnings.rs` under `tests/ui/diagnostic-flags/`
- Improve the test to use two *differential* revisions:
1. One revision checks that without `-A warnings` the code sample
actually emits a warning.
2. The other revision checks that `-A warnings` suppresses the
warning.
This makes sure that if the code sample no longer warns, the test
doesn't silently pass but fail to check its intended purpose.
