Tests/LibWeb: Restructure Ref and Screenshot test dirs to match others

Now each test type has the same directories:
- input
- expected
- data

Also, tests can be in subdirectories within ./input.

Tests/LibWeb/Ref/reference/inline-stacking-context-ref.html

@@ -1,17 +0,0 @@
-<style>
-#text {
-    z-index: 10;
-    background: orange;
-    position: relative;
-    opacity: 0.5;
-    width: fit-content;
-}
-#box {
-    z-index: 5;
-    width: 50px;
-    height: 50px;
-    background: green;
-    position: relative;
-    top: -10px;
-}
-</style><div id="text">hello</div><div id="box"></div>