fix(ci): add @vitest/coverage-v8 to @termui/testing devDependencies

Coverage run passes --coverage to all packages via turbo; testing
package was missing the coverage provider, causing it to fail.

Author: Arindam200

packages/testing/package.json

@@ -27,6 +27,7 @@
   "devDependencies": {
     "@types/react": "^18.0.0",
     "@types/node": "^20.0.0",
+    "@vitest/coverage-v8": "^1.0.0",
     "typescript": "^5.4.0",
     "vitest": "^1.0.0"
   }