Related to #184

Implement a scalable testing system for Rust functions in the Cap desktop application.

• Add test cases: Create a new file apps/desktop/src-tauri/tests/desktop_tests.rs with test cases for start_recording, stop_recording, copy_file_to_path, copy_screenshot_to_clipboard, and export_video.
• Update package.json: Add a new command test:desktop in apps/desktop/package.json to run the Rust tests using cargo test --manifest-path src-tauri/Cargo.toml.

CMD:cargo add --dev tokio --dev anyhow --dev tempfile --dev assert_cmd --dev predicates

/claim #184