sets max workers to speed up tests on CI

jestjs/jest#5239 (comment)

package.json

@@ -21,7 +21,7 @@
     "build:prod": "webpack --config config/webpack.prod.config.js",
     "build:schema-validator": "ajv compile -s src/client/reducers/schema.json -o src/client/reducers/ValidateConfiguration.js --remove-additional --all-errors",
     "test": "jest --colors",
-    "test:coverage": "jest --colors --coverage",
+    "test:coverage": "jest --maxWorkers=2 --colors --coverage",
     "test:journey": "npm run cypress",
     "lint": "npm run lint:js && npm run lint:css",
     "lint:js": "eslint ./src/client ./test/client",