refactor(samples): convert samples test from ava to mocha (#154)

Author: nareshqlogic

packages/google-cloud-dns/samples/package.json

@@ -9,9 +9,7 @@
     "node": ">=8"
   },
   "scripts": {
-    "ava": "ava -T 20s --verbose test/*.test.js system-test/*.test.js",
-    "cover": "nyc --reporter=lcov --cache ava -T 20s --verbose test/*.test.js system-test/*.test.js && nyc report",
-    "test": "npm run cover"
+    "test": "mocha system-test/*.js --timeout 600000"
   },
   "dependencies": {
     "@google-cloud/dns": "^0.8.0",