Add test for suggestion in case a "ignore" doc block has invalid rust code inside

Author: GuillaumeGomez

src/test/rustdoc-ui/ignore-block-help.rs

@@ -0,0 +1,7 @@
+// check-pass
+
+/// ```ignore (to-prevent-tidy-error)
+/// let heart = '❤️';
+/// ```
+//~^^^ WARN
+pub struct X;

src/test/rustdoc-ui/ignore-block-help.stderr

@@ -0,0 +1,17 @@
+warning: could not parse code block as Rust code
+  --> $DIR/ignore-block-help.rs:3:5
+   |
+LL |   /// ```ignore (to-prevent-tidy-error)
+   |  _____^
+LL | | /// let heart = '❤️';
+LL | | /// ```
+   | |_______^
+   |
+   = note: error from rustc: character literal may only contain one codepoint
+help: `ignore` code blocks require valid Rust code for syntax highlighting. Mark blocks that do not contain Rust code as text
+   |
+LL | /// ```text,ignore (to-prevent-tidy-error)
+   |     ^^^^^^^^
+
+warning: 1 warning emitted
+