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/scrollable-box-with-nested-stacking-context-ref.html

@@ -1,32 +0,0 @@
-<style>
-    * {
-        scrollbar-width: none;
-    }
-
-    #container {
-        width: 300px;
-        height: 500px;
-        overflow: auto;
-        position: relative;
-        border: 2px solid black;
-    }
-
-    .content {
-        background-color: lightblue;
-        height: 100%;
-    }
-
-    .box {
-        background-color: magenta;
-        width: 100px;
-        height: 50px;
-        position: absolute;
-        top: 0px;
-        right: 50px;
-    }
-</style>
-<div id="container">
-    <div class="content">
-        <div class="box"></div>
-    </div>
-</div>