Fix unit tests broken by modal bar animation

The modal bar animation added by @larz0 added a bit of asynchronicity to the modal bar: it now isn't removed from the DOM until the close animation completes. This pull request fixes up the unit tests so that they wait for the previous modal bar to close (and forcibly remove it at the end of the test) before continuing.

Also, some unit tests were breaking because the modal bar was never removing itself from the DOM. This was due to a subtle issue: the new modal bar code was using the `.hide` class, but that class is also defined by Bootstrap to set `display: none`. That wasn't affecting the modal bar's behavior in Brackets itself, but was affecting it in the test runner, possibly because stylesheets are loaded in a different order. So I just renamed the class to `.modal-bar-hide` to avoid getting contaminated by the Bootstrap rule.