build: Move tests/reftests to testsuite/reftests

testsuite/reftests/style-properties-only-child.css

@@ -0,0 +1,9 @@
+.only-child-ref:nth-child(first):nth-child(last) {
+    color: blue;
+    background-color: red;
+}
+
+.only-child-notref:only-child {
+    color: blue;
+    background-color: red;
+}