Modify borders ref test to work on OS X

Issue https://github.com/mozilla/servo/issues/2848 has details.
This commit uncomments the borders ref test in basic.list
and modifies the test to work with viewports up to 4096px wide.

As a workaround change the html element background to white,
otherwise pixel (0,0) is black (it should be white since the border
at pixel (0, 0) is white. The bug is likely a regression
in rust-layers.