test: End-to-end WebVTT layout tests (shaka-project#4935)

Many of these are rendered incorrectly in some way.

This PR contains only the tests, but not the screenshots, to make the
tests easier to review. The screenshots will appear in a separate PR.

Related to work on PR shaka-project#4767

test/test/assets/screenshots/review.html

@@ -58,6 +58,39 @@
     'region-with-display-alignment',
     'line-alignment',
   ],
+  'webvtt': [
+    'align-center',
+    'align-center-long',
+    'align-center-position-50',
+    'align-center-position-90',
+    'align-center-position-10',
+    'align-center-position-10-size-10',
+    'align-end',
+    'align-end-long',
+    'align-start',
+    'align-start-long',
+    'align-position-line',
+    'size-50',
+    'line-negative-2',
+    'line-0',
+    'line-1',
+    'line-50',
+    'bold-long',
+    'italic-long',
+    'underline-long',
+    'escaped-entities',
+    'white-spaces',
+    'embedded-styles',
+    'voices',
+    'voices-with-styles',
+    'classes',
+    'classes-with-styles',
+    'region-no-anchors',
+    'region-anchor-top-center',
+    'region-anchor-top-left',
+    'region-complex-anchors',
+    'region-2-lines-scroll-up',
+  ],
 };
 
 const prefixes = [