Groove and Ridge rendering shows a solid border when color is black,
that is broken and the current patch will implement a similar behavior
as Firefox.
Adding tests to check behavior. There are 3 files that will test:
a) If the groove border is different from a black border (the bug that this
patch set fixes);
b) If a ridge border is different from a black border (samething);
c) If groove is different from ridge (they should be inverse of each other).

All 3 new tests passes on OSX Yosemite (10.10).
Restored older behavior for lighter color, should fix
the regression.
auto merge of #4324 : Adenilson/servo/grooveBorderColor01, r=pcwalton

Groove and Ridge rendering shows a solid border when color is black, that is broken and the current patch will implement a similar behavior as Firefox.