Do not run nyc on npm test task. (#570)

Since, nyc 15.0+ is unstable...
istanbuljs/nyc#1263

Author: tokuhirom

package.json

@@ -13,7 +13,8 @@
   ],
   "scripts": {
     "pretest": "npm run format && npm run build",
-    "test": "TEST_PORT=1234 TS_NODE_CACHE=0 nyc mocha",
+    "test": "TEST_PORT=1234 TS_NODE_CACHE=0 mocha",
+    "covtest": "TEST_PORT=1234 TS_NODE_CACHE=0 nyc mocha",
     "prettier": "prettier \"{lib,test}/**/*.ts\"",
     "format": "npm run prettier -- --write",
     "format:check": "npm run prettier -- -l",