Fixes race conditions in functional test introduced in PR#2395

Fixes race conditions in functional test introduced in [PR#2395](https://github.com/TryGhost/Ghost/pull/2395)

fixes [PR#2395](https://github.com/TryGhost/Ghost/pull/2395)
- in editor_test.js line 272, I wait until the post settings menu appears, and THEN also wait until the delete post button appears, rather than just waiting for the first condition, this can result in a race condition which has been seen.
- in editor_test.js line 229, I click on the tag, then wait until it disappears, rather than asserting the tag doesn't exist immediately.  This could potentially result in a race condition, but I have not seen it yet.
- in editr_test.js line 315, I click on the delete post button, and now wait for the modal container to appear before clicking the accept button.  This could result in a race condition, but I have not seen it yet.