Adjust allow-non-lint-warnings.rs

- 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.

tests/ui/allow-non-lint-warnings.rs

@@ -1,8 +0,0 @@
-//@ compile-flags: -Awarnings
-//@ check-pass
-
-#[derive()]
-#[derive(Copy, Clone)]
-pub struct Foo;
-
-pub fn main() {}

tests/ui/diagnostic-flags/allow-non-lint-warnings.rs

@@ -0,0 +1,27 @@
+// ignore-tidy-linelength
+//! Check that `-A warnings` cli flag applies to non-lint warnings as well.
+//!
+//! This test tries to exercise that by checking that the "relaxing a default bound only does
+//! something for `?Sized`; all other traits are not bound by default" non-lint warning (normally
+//! warn-by-default) is suppressed if the `-A warnings` cli flag is passed.
+//!
+//! Take special note that `warnings` is a special pseudo lint group in relationship to non-lint
+//! warnings, which is somewhat special. This test does not exercise other `-A <other_lint_group>`
+//! that check that they are working in the same way, only `warnings` specifically.
+//!
+//! # Relevant context
+//!
+//! - Original impl PR: <https://github.com/rust-lang/rust/pull/21248>.
+//! - RFC 507 "Release channels":
+//!   <https://github.com/rust-lang/rfcs/blob/c017755b9bfa0421570d92ba38082302e0f3ad4f/text/0507-release-channels.md>.
+#![crate_type = "lib"]
+
+//@ revisions: without_flag with_flag
+
+//@[with_flag] compile-flags: -Awarnings
+
+//@ check-pass
+
+pub trait Trait {}
+pub fn f<T: ?Trait>() {}
+//[without_flag]~^ WARN relaxing a default bound only does something for `?Sized`; all other traits are not bound by default

tests/ui/diagnostic-flags/allow-non-lint-warnings.without_flag.stderr

@@ -0,0 +1,8 @@
+warning: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+  --> $DIR/allow-non-lint-warnings.rs:26:13
+   |
+LL | pub fn f<T: ?Trait>() {}
+   |             ^^^^^^
+
+warning: 1 warning emitted
+