Add parser tests for border-image properties

This adds tests for border-image-source, border-image-repeat,
border-image-slice and border-image-width.

testsuite/css/parser/border-image-slice.ref.css

@@ -0,0 +1,51 @@
+a {
+  border-image-slice: initial;
+}
+
+b {
+  border-image-slice: inherit;
+}
+
+c {
+  border-image-slice: unset;
+}
+
+d {
+  border-image-slice: 10;
+}
+
+e {
+  border-image-slice: 10 11;
+}
+
+f {
+  border-image-slice: 10 11 12;
+}
+
+f {
+  border-image-slice: 10 11 12 13;
+}
+
+g {
+  border-image-slice: 10%;
+}
+
+h {
+  border-image-slice: 10% 11%;
+}
+
+i {
+  border-image-slice: 10% 11% 12%;
+}
+
+j {
+  border-image-slice: 10% 11% 12% 13%;
+}
+
+k {
+  border-image-slice: 10% 11% 12% fill;
+}
+
+l {
+  border-image-slice: 10% 11 12 fill;
+}