strapi/test/jest2e2.setup.js

expect.extend({
  stringOrNull(received) {
    const pass = typeof received === 'string' || received === null;
    if (pass) {
      return {
        message: () => `expected ${received} not to be null or a string`,
        pass: true,
      };
    } else {
      return {
        message: () => `expected ${received} to be null or a string`,
        pass: false,
      };
    }
  },
});