ember-mocha: Component tests run fine if in the 'root' describe block, but throw 'You have turned on testing mode, which disabled the run-loop's autorun' error when in nested describe blocks

The following pseudo code works fine:

describe('Unit | Component | common | input-with-validation', function() {
  setupComponentTest('componentToTest', {
    needs: [],
    unit: true
  });
  beforeEach(function () {
    component = this.subject();
  });

  it('Test Observer', function() {
    component.set('observedKey', true);
  }
}

but the following does not, and raises an assertion error: You have turned on testing mode, which disabled the run-loop's autorun. You will need to wrap any code with asynchronous side-effects in a run

describe('Unit | Component | common | input-with-validation', function() {
  setupComponentTest('componentToTest', {
    needs: [],
    unit: true
  });

  beforeEach(function () {
    component = this.subject();
  });

  describe('Observers', function () {
    it('Test Observer', function() {
      component.set('observedKey', true);
    }
  }
}