Add TestWork functionality to preserve working directories on failure (#24)

- [x] Analyze current codebase and understand test execution flow
- [x] Add preserve_work_on_failure field to RunParams struct  
- [x] Add preserve_work_on_failure() method to Builder
- [x] Modify TestEnvironment to support preserving work directories
- [x] Update run_script_impl to handle work directory preservation on
failure
- [x] Add comprehensive tests to validate the new functionality  
- [x] Update documentation with usage examples
- [x] Update README with TestWork section showing debugging capabilities
- [x] Fix clippy warnings for code quality
- [x] Validate functionality works correctly in both debug and release
modes
- [x] Fix formatting issues that caused CI failure
- [x] Replace std::mem::forget() with idiomatic TempDir::keep() method

## Improved Code Quality ✅

Replaced the non-idiomatic use of `std::mem::forget()` with the proper
`TempDir::keep()` method for preserving temporary directories. This
approach:

- Is more explicit about the intent to preserve the directory
- Is the recommended way according to tempfile crate documentation  
- Removes the need for manual memory management workarounds
- Makes the code more readable and maintainable

All tests continue to pass and functionality remains unchanged.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Add TestWork functionality to preserve working
directories</issue_title>
> <issue_description>## Feature Request: TestWork Functionality
> 
> Go's testscript supports a `TestWork` parameter that preserves working
directories when tests fail, making debugging much easier.
> 
> ## Proposed API
> 
> ```rust
> testscript::run("testdata")
>     .preserve_work_on_failure(true)
>     .execute()
>     .unwrap();
> ```
> 
> ## Implementation
> 
> - When a test fails and this option is enabled, print the work
directory path
> - Don't clean up the temporary directory on test failure
> - Could also support preserving on success for debugging
> 
> ## Benefits
> 
> - **Easier debugging**: Inspect files created during failed tests
> - **Development workflow**: Understand what went wrong
> - **Compatibility**: Matches Go testscript behavior
> 
> ## Example Output
> 
> ```
> Test failed. Work directory preserved at: /tmp/testscript-work-abc123
> You can inspect the test environment:
>   cd /tmp/testscript-work-abc123
>   ls -la
> ```</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>
Fixes imjasonh/testscript-rs#6

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: imjasonh <210737+imjasonh@users.noreply.github.com>
Co-authored-by: Jason Hall <jason@chainguard.dev>

examples/test_runner.rs

@@ -8,8 +8,22 @@ fn main() {
         std::env::set_current_dir(manifest_dir).expect("Failed to change directory");
     }
 
-    // Run all tests in testdata directory
-    match testscript::run("testdata").execute() {
+    // Check for test directory from environment for test purposes
+    let test_dir = std::env::var("TESTSCRIPT_TEST_DIR").unwrap_or_else(|_| "testdata".to_string());
+
+    // Check if preserve work should be enabled
+    let preserve_work = std::env::var("TESTSCRIPT_PRESERVE_WORK")
+        .map(|v| v == "true" || v == "1")
+        .unwrap_or(false);
+
+    // Run all tests in the specified directory
+    let mut builder = testscript::run(test_dir);
+
+    if preserve_work {
+        builder = builder.preserve_work_on_failure(true);
+    }
+
+    match builder.execute() {
         Ok(()) => println!("All tests passed!"),
         Err(e) => {
             eprintln!("Test failed: {}", e);