Using the mixinJestEnvironment mixin results in failing jest tests during normal unit testing

[nicojs]

Summary

Using the mixin described in the docs results in an error during normal unit testing. I think this bug got introduced in Stryker v6.

Reproduction

In the StrykerJS repo:

cd e2e/test/jest-custom-env
npm run test:unit

This results in:

npm run test:unit

> test:unit
> jest

 FAIL  src/calculator-element.test.js
  ● Test suite failed to run

    TypeError: Cannot assign to read only property 'undefined' of object '[object global]'

      at new StrykerJestEnvironment (../../node_modules/@stryker-mutator/jest-runner/dist/src/jest-plugins/cjs/mixin-jest-environment.js:41:91)