Add ui test for rustdoc broken_footnote lint

Author: GuillaumeGomez

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

@@ -0,0 +1,7 @@
+#![deny(rustdoc::broken_footnote)]
+
+//! Footnote referenced [^1]. And [^2]. And [^bla].
+//!
+//! [^1]: footnote defined
+//~^^^ ERROR: no footnote definition matching this footnote
+//~| ERROR: no footnote definition matching this footnote

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

@@ -0,0 +1,24 @@
+error: no footnote definition matching this footnote
+  --> $DIR/broken-footnote.rs:3:35
+   |
+LL | //! Footnote referenced [^1]. And [^2]. And [^bla].
+   |                                   -^^^
+   |                                   |
+   |                                   help: if it should not be a footnote, escape it: `\`
+   |
+note: the lint level is defined here
+  --> $DIR/broken-footnote.rs:1:9
+   |
+LL | #![deny(rustdoc::broken_footnote)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: no footnote definition matching this footnote
+  --> $DIR/broken-footnote.rs:3:45
+   |
+LL | //! Footnote referenced [^1]. And [^2]. And [^bla].
+   |                                             -^^^^^
+   |                                             |
+   |                                             help: if it should not be a footnote, escape it: `\`
+
+error: aborting due to 2 previous errors
+