Add ui test for new unused_footnote_definition rustdoc lint

Author: GuillaumeGomez

tests/rustdoc-ui/lints/unused-footnote.rs

@@ -0,0 +1,9 @@
+// This test ensures that the rustdoc `unused_footnote` is working as expected.
+
+#![deny(rustdoc::unused_footnote_definition)]
+
+//! Footnote referenced. [^2]
+//!
+//! [^1]: footnote defined
+//! [^2]: footnote defined
+//~^^ unused_footnote_definition

tests/rustdoc-ui/lints/unused-footnote.stderr

@@ -0,0 +1,14 @@
+error: unused footnote definition
+  --> $DIR/unused-footnote.rs:7:6
+   |
+LL | //! [^1]: footnote defined
+   |      ^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-footnote.rs:3:9
+   |
+LL | #![deny(rustdoc::unused_footnote_definition)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+