LibWeb: Add simple ref test for SVG masking

Tests/LibWeb/Ref/reference/simple-svg-mask-ref.html

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+<svg width="200" viewBox="-10 -10 120 120">
+  <rect x="-10" y="-10" width="120" height="120" fill="blue" />
+  <rect x="50" y="10" width="40" height="80" fill="red"  />
+</svg>

Tests/LibWeb/Ref/simple-svg-mask.html

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+<link rel="match" href="reference/simple-svg-mask-ref.html" />
+<svg width="200" viewBox="-10 -10 120 120">
+  <mask id="myMask">
+    <!-- Everything under a white pixel will be visible -->
+    <rect x="0" y="0" width="100" height="100" fill="white" />
+    <!-- Everything under a black pixel will be invisible -->
+    <rect x="0" y="0" width="50" height="100" fill="black" />
+  </mask>
+  <rect x="-10" y="-10" width="120" height="120" fill="blue" />
+  <rect x="10" y="10" width="80" height="80" fill="red" mask="url(#myMask)" />
+</svg>

Tests/LibWeb/Ref/svg-mask-in-defs.html

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+<link rel="match" href="reference/simple-svg-mask-ref.html" />
+<svg width="200" viewBox="-10 -10 120 120">
+  <defs>
+    <mask id="myMask">
+      <!-- Everything under a white pixel will be visible -->
+      <rect x="0" y="0" width="100" height="100" fill="white" />
+      <!-- Everything under a black pixel will be invisible -->
+      <rect x="0" y="0" width="50" height="100" fill="black" />
+    </mask>
+  </defs>
+  <rect x="-10" y="-10" width="120" height="120" fill="blue" />
+  <rect x="10" y="10" width="80" height="80" fill="red" mask="url(#myMask)" />
+</svg>