Review possibility to add `test_case` attribute to items within `impl` blocks

As in title. Since we support using test case on free floating `fn` items, it would be useful to review if we can enable this functionality on `impl` blocks, eg.:
```rust
impl StructName {
#[test_case("abcd" => Ok(("ab", "cd")))]
fn parse(input: &[u8]) -> IResult<&[u8], &[u8])> {
todo!()
}
}
```
this may require extra attribute though.