Rollup merge of #92959 - asquared31415:test-non-fn-help, r=estebank

Add more info and suggestions to use of #[test] on invalid items

This pr changes the diagnostics for using `#[test]` on an item that can't be used as a test to explain that the attribute has no meaningful effect on non-functions and suggests the use of `#[cfg(test)]` for conditional compilation instead.

Example change:
```rs
#[test]
mod test {}
```
previously output
```
error: only functions may be used as tests
 --> src/lib.rs:2:1
  |
2 | mod test {}
  | ^^^^^^^^^^^
  ```
  now outputs
  ```
error: the `#[test]` attribute may only be used on a non-associated function
  --> $DIR/test-on-not-fn.rs:3:1
     |
LL | #[test]
     | ^^^^^^^
LL | mod test {}
     | ----------- expected a non-associated function, found a module
     |
     = note: the `#[test]` macro causes a a function to be run on a test and has no effect on non-functions
help: replace with conditional compilation to make the item only exist when tests are being run
     |
LL | #[cfg(test)]
     | ~~~~~~~~~~~~
   ```

0 files changed, 0 insertions, 0 deletions