Mirage currently starts automatically (when not disabled in your application config) in acceptance tests only. This is because Mirage’s server boots up in an Ember initializer.
You can still start a Mirage server for your integration and unit tests with the following workaround:
// tests/integration/components/your-test.js
import { startMirage } from 'yourapp/initializers/ember-cli-mirage';
moduleForComponent('your-component', 'Integration | Component | your component', {
  integration: true,
  beforeEach() {
    this.server = startMirage();
  },
  afterEach() {
    this.server.shutdown();
  }
});
Some notes:
dummy/initializers/ember-cli-mirage.urlPrefix and namespace may require to be adapted when used with integration or unit tests.