Access test case name from test case body

Example code

```rust
#[test_case(vec![(1,1),(2,2),(3,3)].iter().collect(), vec![(1,1),(2,2),(3,3)].iter().collect() ; "test")]
fn get_changes_values<
'a,
'b,
K: Eq + Ord + Clone + fmt::Debug,
V: Eq + fmt::Debug,
>(
old: &'b BTreeMap<&'a K, V>,
new: &'b BTreeMap<&'a K, V>,
) {
let changes = super::get_changes_values(old, new);
// test_case => "test"
assert_debug_snapshot(test_case, changes);
}
```

Start with the test_case procedural macro implementation and trace how the attribute body is parsed and expanded. Use the example in the issue as the behavioral reference; done means the generated test body can access the case name as shown by the snapshot assertion.

Written by the indexing model from the issue text.