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)).

docs/src/tasks.md

@@ -43,10 +43,12 @@ These variables allow you to pull information from the current editor and use it
 
 - `ZED_COLUMN`: current line column
 - `ZED_ROW`: current line row
-- `ZED_FILE`: absolute path to the file
+- `ZED_FILE`: absolute path of the currently opened file (e.g. `/Users/my-user/path/to/project/src/main.rs`)
+- `ZED_FILENAME`: filename of the currently opened file (e.g. `main.rs`)
+- `ZED_STEM`: stem (filename without extension) of the currently opened file (e.g. `main`)
 - `ZED_SYMBOL`: currently selected symbol; should match the last symbol shown in a symbol breadcrumb (e.g. `mod tests > fn test_task_contexts`)
 - `ZED_SELECTED_TEXT`: currently selected text
-- `ZED_WORKTREE_ROOT`: absolute path to the root of the current worktree.
+- `ZED_WORKTREE_ROOT`: absolute path to the root of the current worktree. (e.g. `/Users/my-user/path/to/project`)
 - `ZED_CUSTOM_RUST_PACKAGE`: (Rust-specific) name of the parent package of $ZED_FILE source file.
 
 To use a variable in a task, prefix it with a dollar sign (`$`):