Introduce screenshot tests

This patch introduces a goldentest.js. The tests inside
the file should rely on "golden" results rather then asserts.

For now, goldentest.js allows only image expectations. If the
actual result doesn't match the expected result, the two files
are created under `test/output` folder:
- The '-actual.png' contains the actual test result
- The '-diff.png' contains the diff between images

test/assets/grid.html

@@ -0,0 +1,44 @@
+<script>
+document.addEventListener('DOMContentLoaded', function() {
+    function generatePalette(amount) {
+        var result = [];
+        var hueStep = 360 / amount;
+        for (var i = 0; i < amount; ++i)
+            result.push('hsl(' + (hueStep * i) + ', 100%, 90%)');
+        return result;
+    }
+
+    var palette = generatePalette(100);
+    for (var i = 0; i < 1000; ++i) {
+        var box = document.createElement('div');
+        box.classList.add('box');
+        box.textContent = i;
+        box.style.setProperty('background-color', palette[i % palette.length]);
+        document.body.appendChild(box);
+    }
+});
+</script>
+
+<style>
+
+/* Hide scrollbar so that it is not captured on screenshots */
+::-webkit-scrollbar {
+    display: none;
+}
+
+body {
+    margin: 0;
+    padding: 0;
+}
+
+.box {
+    font-family: arial;
+    display: inline-block;
+    margin: 0;
+    padding: 0;
+    width: 50px;
+    height: 50px;
+    box-sizing: border-box;
+    border: 1px solid darkgray;
+}
+</style>