Tests: Add slider visual test to the index page

(cherry picked from commit 65f4feb)

Author: tjvantoll

tests/visual/index.html

@@ -68,6 +68,11 @@ <h2>Selectmenu</h2>
 			<li><a href="selectmenu/selectmenu.html">General</a></li>
 		</ul>
 
+		<h2>Slider</h2>
+		<ul>
+			<li><a href="slider/range_slider.html">Range slider</a></li>
+		</ul>
+
 		<h2>Tooltip</h2>
 		<ul>
 			<li><a href="tooltip/tooltip.html">General</a></li>