fix: update mocha to ^8.4.0

Author: buffcode

package.json

@@ -25,7 +25,7 @@
   "devDependencies": {
     "@types/mocha": "^8.2.2",
     "@types/node": "^15.3.0",
-    "mocha": "^3.5.0",
+    "mocha": "^8.4.0",
     "prettier": "^1.6.1",
     "ts-mocha": "^8.0.0",
     "ts-node": "^9.1.1",