Change the hiding strategy of the test_iframe.

This commit contains three changes:

  * 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 inttroduced is a configurable delay for when the test_iframe
    actally considers a test page being done loading and signaling the
    test-suite that it can continue with running the tests.
Remove no longer needed test-utility.js.

Since the previous commit (which changed the way the test_iframe is
hidden from the user), we no longer need to manually call the utility
methods `showTestIframe` and `hideTestIframe`.

Since these methods were the only ones contained in test-utility.js,
that file is now obsolete.
Merge pull request #319 from marcjansen/test-iframe-hide-strategy

Change the way we hide the test iframe