Improve browser test shell

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.