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-width.css

@@ -0,0 +1,71 @@
+a {
+  border-image-width: initial;
+}
+
+b {
+  border-image-width: inherit;
+}
+
+c {
+  border-image-width: unset;
+}
+
+d {
+  border-image-width: 1 1 1 1;
+}
+
+e {
+  border-image-width: 1;
+}
+
+f {
+  border-image-width: 1 2;
+}
+
+g {
+  border-image-width: 1 2 3;
+}
+
+h {
+  border-image-width: 1 2 3 4;
+}
+
+i {
+  border-image-width: 1px;
+}
+
+j {
+  border-image-width: 1px 2px;
+}
+
+k {
+  border-image-width: 1px 2px 3px;
+}
+
+l {
+  border-image-width: 1px 2px 3px 4px;
+}
+
+m {
+  border-image-width: 1%;
+}
+
+n {
+  border-image-width: 1% 2%;
+}
+
+o {
+  border-image-width: 1% 2% 3%;
+}
+
+p {
+  border-image-width: 1% 2% 3% 4%;
+}
+
+q {
+  border-image-width: auto;
+}
+
+r {
+  border-image-width: 1 2px 3% auto;
+}