CI: update mocha (#1503)

Update to mocha@8 instead of 9 because the latter dropped support for `--require
esm`, hence using it would require more extensive changes.

package.json

@@ -67,7 +67,7 @@
     "babel-eslint": "^10.1.0",
     "benchmark": "^2.1.4",
     "bn.js": "^4.11.8",
-    "chai": "^4.1.2",
+    "chai": "^4.3.6",
     "chai-as-promised": "^7.1.1",
     "email-addresses": "3.1.0",
     "eslint": "^4.17.0",
@@ -78,11 +78,11 @@
     "esm": "^3.2.25",
     "hash.js": "^1.1.3",
     "http-server": "^0.12.3",
-    "karma": "^6.1.0",
+    "karma": "^6.3.17",
     "karma-browserstack-launcher": "^1.6.0",
     "karma-mocha": "^2.0.1",
     "karma-mocha-reporter": "^2.2.5",
-    "mocha": "^5.0.0",
+    "mocha": "^8.4.0",
     "nyc": "^14.1.1",
     "rollup": "^2.38.5",
     "rollup-plugin-terser": "^7.0.2",