Add support for running specific test files (#32)

Adds a new `.files()` method to the Builder API that allows running
specific test files instead of discovering all `.txt` files in a
directory. This mirrors the functionality of Go's testscript `Files`
parameter.

## New API

```rust
// Run only specific test files
testscript::run("testdata")
    .files(["hello.txt", "exists.txt"])
    .execute()
    .unwrap();

// Combine with other features
testscript::run("testdata")
    .files(["hello.txt", "exists.txt"])
    .setup(|env| { /* setup code */ Ok(()) })
    .condition("custom", true)
    .preserve_work_on_failure(true)
    .execute()
    .unwrap();
```

## Benefits

- **Selective testing**: Run only specific test scenarios during
development
- **Faster CI**: Run subsets of tests in parallel jobs
- **Go compatibility**: Matches `testscript.Run(t,
testscript.Params{Files: []string{...}})`
- **Flexible paths**: Supports relative paths, absolute paths, and
filenames

## Implementation Details

- Adds optional `files` field to `RunParams` 
- Enhances internal `run()` function to handle file lists vs glob
discovery
- Preserves execution order as specified by user
- Comprehensive error handling for missing files and invalid paths
- Full backward compatibility - existing code unchanged

## Path Support

```rust
testscript::run("testdata")
    .files([
        "hello.txt",              // filename in testdata dir
        "subdir/nested.txt",      // relative path  
        "/abs/path/to/test.txt"   // absolute path
    ])
    .execute()
    .unwrap();
```

The feature includes focused test coverage with 7 test cases covering
essential scenarios including error conditions and path types. Based on
feedback, redundant tests were removed to maintain a clean and
meaningful test suite.

Fixes imjasonh/testscript-rs#7

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Add support for running specific test files</issue_title>
> <issue_description>## Feature Request: Run Specific Test Files
> 
> Go's testscript supports running specific files via the `Files`
parameter:
> 
> ```go
> testscript.Run(t, testscript.Params{
>     Files: []string{"specific_test.txt", "another_test.txt"},
> })
> ```
> 
> ## Proposed API
> 
> ```rust
> testscript::run("testdata")
>     .files(&["hello.txt", "exists.txt"])
>     .execute()
>     .unwrap();
> ```
> 
> ## Benefits
> 
> - **Selective testing**: Run only specific test scenarios
> - **Faster development**: Skip unrelated tests during development
> - **CI optimization**: Run subsets of tests in different jobs
> - **Go compatibility**: Match Go testscript functionality
> 
> ## Implementation
> 
> - Extend the Builder API with a `.files()` method
> - Create a separate `run_files()` entry point
> - Support both absolute and relative paths
> - Validate that specified files exist</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>
Fixes imjasonh/testscript-rs#7

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src/run/params.rs

@@ -24,6 +24,8 @@ pub struct RunParams {
     pub preserve_work_on_failure: bool,
     /// Optional root directory for test working directories
     pub workdir_root: Option<std::path::PathBuf>,
+    /// Specific files to run (if None, discover all .txt files)
+    pub files: Option<Vec<String>>,
 }
 
 impl RunParams {
@@ -58,6 +60,7 @@ impl RunParams {
             update_scripts,
             preserve_work_on_failure: false,
             workdir_root: None,
+            files: None,
         }
     }
 
@@ -103,6 +106,19 @@ impl RunParams {
         self
     }
 
+    /// Set specific files to run instead of discovering all .txt files
+    ///
+    /// When specified, only these files will be executed instead of discovering
+    /// all .txt files in the directory.
+    pub fn files<I, S>(mut self, files: I) -> Self
+    where
+        I: IntoIterator<Item = S>,
+        S: Into<String>,
+    {
+        self.files = Some(files.into_iter().map(|s| s.into()).collect());
+        self
+    }
+
     /// Check if a program exists in PATH (cross-platform)
     pub fn program_exists(program: &str) -> bool {
         // TODO: Consider caching results for performance if needed