refactor: use execSync for tests (#127)

Author: JustinBeckwith

asset/snippets/package.json

@@ -12,7 +12,7 @@
   "repository": "googleapis/nodejs-asset",
   "private": true,
   "scripts": {
-    "test": "mocha system-test --timeout 20000"
+    "test": "mocha --timeout 20000"
   },
   "dependencies": {
     "@google-cloud/asset": "^0.3.0",
@@ -22,7 +22,6 @@
   },
   "devDependencies": {
     "chai": "^4.2.0",
-    "execa": "^1.0.0",
     "mocha": "^6.0.0"
   }
 }