rust: Add runnable icon for #[cfg(test)] mod tests (#12017)

This allows you to run all the tests inside the `mod tests` block. 
Fixes #11967.

<img width="366" alt="Screenshot 2024-05-18 at 16 07 32"
src="https://github.com/zed-industries/zed/assets/62463826/01fc378c-1546-421d-8250-fe0227c1e5a0">

<img width="874" alt="Screenshot 2024-05-18 at 16 37 21"
src="https://github.com/zed-industries/zed/assets/62463826/4a880b91-df84-4917-a16e-5d5fe20e22ac">

Release Notes:

- Added runnable icon for Rust `#[cfg(test)] mod tests` blocks
([#11967](https://github.com/zed-industries/zed/issues/11967)).

crates/languages/src/rust/runnables.scm

@@ -1,3 +1,8 @@
+(mod_item
+    name: (_) @run
+    (#eq? @run "tests")
+) @rust-mod-test
+
 (
     (attribute_item (attribute
         [((identifier) @_attribute)