Rollup merge of #121580 - Suyashtnt:issue-121502-fix, r=michaelwoerister

make unused_imports less assertive in test modules

closes #121502

This is a fairly small change and I used the fix suggested in the example expected error message.
Not sure if I should've rather used the alternatives but this one seems the most descriptive.

Some alternatives:
- if this is meant to be a test module, add `#[cfg(test)]` to the containing module
- try adding #[cfg(test)] to this test module
- consider adding #[allow(unused_imports)] if you want to silent the lint on the unused import
- consider removing the unused import

compiler/rustc_lint/src/context/diagnostics.rs

@@ -104,7 +104,7 @@ pub(super) fn builtin(sess: &Session, diagnostic: BuiltinLintDiagnostics, diag:
             if let Some(span) = in_test_module {
                 diag.span_help(
                     sess.source_map().guess_head_span(span),
-                    "consider adding a `#[cfg(test)]` to the containing module",
+                    "if this is a test module, consider adding a `#[cfg(test)]` to the containing module",
                 );
             }
         }