Bug 990481 - [ZTE][OPENC]MMS app allows to attach more contents even whe...

...n the max MMS size is reached
Looks good, but there is no need in such small tests, you can just join it like below 

```
suite('lock() and unlock()',function() {
  test('correctly manages state of attachment button', function() {
    var attachButton = document.getElementById('messages-attach-button');

    // Should be enabled at the beginning
    assert.isFalse(attachButton.disabled);

    Compose.lock();

    // Lock should disable attachment button
    assert.isTrue(attachButton.disabled);

    Compose.unlock();

    // Unlock should enable attachment button again
    assert.isFalse(attachButton.disabled);
  });
});
```