Bug 1397619: Test border-spacing specified value serialization. r=me

MozReview-Commit-ID: 1L5mjfvYRLQ
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

layout/style/test/test_specified_value_serialization.html

@@ -261,6 +261,7 @@
     "border-bottom-left-radius",
     "border-top-right-radius",
     "border-bottom-right-radius",
+    "border-spacing",
   ].forEach(function(property) {
     for (var i = 0; i < borderRadius.length; ++i) {
       var test = borderRadius[i];