Improve browser test shell

[brendandahl]

With small browser windows the spinner, progress bar, and status text shift around causing the canvas below them to be at a different location from when the test starts. Ideally these things wouldn't cause the layout shift around. This can be observed in browser.test_sdl_mouse when using a small window.

Some options:

1. Use a test only shell.html that has the minimal amount elements.
2. Change shell.html to have a fixed size top bar that doesn't affect the content below.

[sbc100]

Another option: Ensure that all DOM manipulation relating to these elements happens before main is run.