Change the way we hide the test iframe

This PR suggests changing the way we hide the frame where `Test.AnotherWay` loads the individual test files. 

Previously this `iframe` was hidden because it was a child of a `span` with `display:none`. This PR changes three things:

  * First, the `test_iframe` is taken out of `span` with `display: none`, so that it is initially visible
  * That `iframe` is hidden via absolute positioning now and negative `top` and `left` values
  * Also introduced is a configurable delay for when the `test_iframe` actually considers a test page being done loading and signaling the test-suite that it can continue with running the tests.

This change then allows us to remove all calls to `showTestIframe()` and `hideTestIframe()`, and eventually to completely remove the `test-utility.js` helper.

All tests pass in FF 33 (Linux), Chromium 38 (Linux), IE 9 (Win 7) and also when executed via travis:

![ie9-win7-geoext2-test](https://cloud.githubusercontent.com/assets/227934/5214820/465649c4-7621-11e4-8677-d53fe60f1996.png)

![tests-ff](https://cloud.githubusercontent.com/assets/227934/5214823/4c40088e-7621-11e4-9d83-edd663ee60dd.png)

![tests-in-chrome-38](https://cloud.githubusercontent.com/assets/227934/5214824/50f0a852-7621-11e4-9463-097f197e7b83.png)

This PR also makes the new test in #312 pass. Once this PR is merged, #312 can be rebased.

Additional information: This behaviour has also been fixed in the `extjs5` branch, see 90f7df792b5375d0ad760cbb0ac201fab0ca2a65. When that particular branch gets merged, that commit will no longer be needed over there.

Please review.