Add macro expansion test for raw variable names

[markmurphydev]

Adds a test covering #17785. Should close that issue.
This seemed like the right location for the test.

The test correctly fails on release 2024-07-15, with:

---- macro_expansion_tests::mbe::matching::meta_variable_raw_name_equals_non_raw stdout ----
thread 'macro_expansion_tests::mbe::matching::meta_variable_raw_name_equals_non_raw' panicked at crates/hir-def/src/macro_expansion_tests/mbe/matching.rs:190:5:
parse errors in expansion:
[
    SyntaxError(
        "expected expression, item or let statement",
        0..0,
    ),
]
```
$name
```