Additional tests for Element#makeClipping(), Element#undoClipping()

[victor-homyakov]

See ticket #1063:

1. Current implementation doesn't work properly for inline style="overflow:auto;"
2. Inline style isn't restored properly after undoClipping() - it is replaced with computed style
3. Computing style with Element.getStyle() isn't needed - performance optimization is possible

[savetheclocktower]

I'll use these tests to help me fix #1063. Until then, I'll keep this open because I don't want to commit tests without the fix that will make them pass. Thanks!