tests: don't ignore compiler stderr in lib-defaults.rs

And also:

- Document test intent.
- Move under `link-native-libs/` instead.

tests/ui/abi/lib-defaults.rs

@@ -1,15 +0,0 @@
-//@ run-pass
-//@ dont-check-compiler-stderr (rust-lang/rust#54222)
-
-//@ compile-flags: -lrust_test_helpers
-
-#[link(name = "rust_test_helpers", kind = "static")]
-extern "C" {
-    pub fn rust_dbg_extern_identity_u32(x: u32) -> u32;
-}
-
-fn main() {
-    unsafe {
-        rust_dbg_extern_identity_u32(42);
-    }
-}

tests/ui/link-native-libs/lib-defaults.rs

@@ -0,0 +1,17 @@
+//! By default, `-l NAME` without an explicit kind will default to dylib. However, if there's also
+//! an `#[link(name = NAME, kind = KIND)]` attribute with an explicit `KIND`, it should override the
+//! CLI flag. In particular, this should not result in any duplicate flag warnings from the linker.
+
+//@ run-pass
+//@ compile-flags: -lrust_test_helpers
+
+#[link(name = "rust_test_helpers", kind = "static")]
+extern "C" {
+    pub fn rust_dbg_extern_identity_u32(x: u32) -> u32;
+}
+
+fn main() {
+    unsafe {
+        rust_dbg_extern_identity_u32(42);
+    }
+}