test: fast tests can no longer be run in parallel

For some reason the tests keep failing with the following error:

```
  1) Uncaught error outside test suite:
     Uncaught Workerpool Worker terminated Unexpectedly
    exitCode: `0`
    signalCode: `null`
    workerpool.script: `electron-forge/node_modules/mocha/lib/nodejs/worker.js`
    spawnArgs: `electron-forge/node_modules/mocha/lib/nodejs/worker.js`
    spawnfile: `/usr/bin/node`
    stdout: `null`
    stderr: `null`
```

package.json

@@ -33,7 +33,7 @@
     "docs:deploy:ci": "ts-node tools/sync-readmes.ts && bolt docs && ts-node tools/copy-now.ts && cd docs && now --token $NOW_TOKEN && now alias --token $NOW_TOKEN",
     "lint": "eslint --ext .ts .",
     "test": "cross-env TS_NODE_FILES=true yarn run mocha './tools/test-globber.ts'",
-    "test:fast": "cross-env TEST_FAST_ONLY=1 TS_NODE_FILES=true yarn run mocha --parallel './tools/test-globber.ts'",
+    "test:fast": "cross-env TEST_FAST_ONLY=1 TS_NODE_FILES=true yarn run mocha './tools/test-globber.ts'",
     "postinstall": "rimraf node_modules/.bin/*.ps1 && ts-node tools/link-ts.ts"
   },
   "dependencies": {