Add unit tests for Trix.MutationObserver
user
```coffeescript
{assert, defer, test, testGroup} = Trix.TestHelpers

observer = null
element = null
summaries = []

install = (html) ->
  element = document.createElement("div")
  element.innerHTML = html if html
  observer = new Trix.MutationObserver element
  observer.delegate =
    elementDidMutate: (summary) ->
      summaries.push(summary)

uninstall = ->
  observer?.stop()
  observer = null
  element = null
  summaries = []

observerTest = (name, options = {}, callback) ->
  test name, (done) ->
    install(options.html)
    callback ->
      uninstall()
      done()


testGroup "Trix.MutationObserver", ->
  observerTest "add character", html: "a", (done) ->
    element.firstChild.data += "b"
    defer ->
      assert.equal summaries.length, 1
      assert.deepEqual summaries[0], textAdded: "b"
      done()

  observerTest "remove character", html: "ab", (done) ->
    element.firstChild.data = "a"
    defer ->
      assert.equal summaries.length, 1
      assert.deepEqual summaries[0], textDeleted: "b"
      done()

  observerTest "replace character", html: "ab", (done) ->
    element.firstChild.data = "ac"
    defer ->
      assert.equal summaries.length, 1
      assert.deepEqual summaries[0], textAdded: "c", textDeleted: "b"
      done()

  observerTest "add <br>", html: "a", (done) ->
    element.appendChild(document.createElement("br"))
    defer ->
      assert.equal summaries.length, 1
      assert.deepEqual summaries[0], textAdded: "\n"
      done()

  observerTest "remove <br>", html: "a<br>", (done) ->
    element.removeChild(element.lastChild)
    defer ->
      assert.equal summaries.length, 1
      assert.deepEqual summaries[0], textDeleted: "\n"
      done()

  observerTest "remove block comment", html: "<div><!--block-->a</div>", (done) ->
    element.firstChild.removeChild(element.firstChild.firstChild)
    defer ->
      assert.equal summaries.length, 1
      assert.deepEqual summaries[0], textDeleted: "\n"
      done()
```
assistant