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.

Also 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 mostly likely a regression
in rust-layers.
Merge pull request #2874 from bjwbell/borders-ref-test

Modify borders ref test to work on OS X