Throw Error When Using Nested It Specs

[rafaelcalpena]

Summary

I have recently started using Jest and I really like it. I tried adding nested it/test conditions, but I didn't realize that was not allowed, because both tests will succeed (nested test, however, never runs). I've added some changes to throw an error to let the devs know that such pattern is not allowed.

Test plan

test('make sure 2 equals 3', () => {
  test('nested test confuses the devs', () => {
    expect(2).toBe(3);
  })
});

 FAIL  ./jest-it.test.js
  ● make sure 2 equals 3

    Test `nested test confuses the devs` cannot run because it is nested in `make sure 2 equals 3`

      at Env.it (../jest/packages/jest-jasmine2/build/jasmine/Env.js:396:15)
      at Object.<anonymous>.test (jest-it.test.js:4:3)
          at Promise (<anonymous>)
          at <anonymous>
      at process._tickCallback (internal/process/next_tick.js:169:7)

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