Fix failing tests on Node v12

The issue of tests failing on Node v12 seems to be related to mocha.
Running it as node node_modules/mocha/bin/_mocha solves the issue.

Author: kyarik

package.json

@@ -41,7 +41,7 @@
     "test": "npm run lint && npm run check && npm run testonly:cover && npm run prettier:check && npm run check:spelling && npm run build:npm && npm run check:integrations",
     "lint": "eslint .",
     "check": "tsc --noEmit",
-    "testonly": "mocha src/**/__tests__/**/*.ts",
+    "testonly": "node node_modules/mocha/bin/_mocha src/**/__tests__/**/*.ts",
     "testonly:cover": "nyc npm run testonly",
     "prettier": "prettier --write --list-different .",
     "prettier:check": "prettier --check .",