This adds two tests:
1. browser_fontinspector_edit-previews.js checks that
* if the text is changed, the preview images should change
* if the text field is empty, the initial previews should be shown
It doesn't check the exact preview images because they are drawn on a canvas
causing them to vary between systems, platforms and software versions.
2. browser_fontinspector_edit-previews-show-all.js checks that the 'Show all'
mode is remembered if the preview text is edited after the button is clicked.
This patch adds a new method openFontInspectorForURL to the head.js file which
a) opens a new tab with the given URL
b) opens inspector for the tab
c) selects font-inspector and waits for it to initialize
Previously the only font-inspector test was doing all that inside the test but
as new tests require the similar functionality it's better to reside in
head.js.
