Explain filtering unit tests by file

Author: Tyriar

.github/copilot-instructions.md

@@ -21,6 +21,7 @@ npm run build && npm run esbuild # Build all TypeScript and bundle
 
 **Testing**:
 - Unit tests: `npm run test-unit` (Mocha)
+- Unit tests filtering to file: `npm run test-unit -- **/fileName.ts
 - Per-addon unit tests: `npm run test-unit addons/addon-image/out-esbuild/*.test.js`
 - Integration tests: `npm run test-integration` (Playwright across Chrome/Firefox/WebKit)
 - Per-addon integration tests: `npm run test-integration --suite=addon-search`