rustdoc: Add test for fixed issue

src/test/rustdoc-ui/issue-61732.rs

+// This previously triggered an ICE.
+
+pub(in crate::r#mod) fn main() {}
+//~^ ERROR expected module, found unresolved item

src/test/rustdoc-ui/issue-61732.stderr

+error[E0577]: expected module, found unresolved item `crate::r#mod`
+  --> $DIR/issue-61732.rs:3:8
+   |
+LL | pub(in crate::r#mod) fn main() {}
+   |        ^^^^^^^^^^^^ not a module
+
+error: Compilation failed, aborting rustdoc
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0577`.