Add tests for on attribute handling

Related ampproject#203

tests/test-amp-blacklist-sanitizer.php

@@ -33,11 +33,16 @@ public function get_data() {
 				'<a href="/path/to/file.jpg">Link</a>'
 			),
 
-			'on_attribute' => array(
+			'onclick_attribute' => array(
 				'<a href="/path/to/file.jpg" onclick="alert(e);">Link</a>',
 				'<a href="/path/to/file.jpg">Link</a>'
 			),
 
+			'on_attribute' => array(
+				'<button on="tap:my-lightbox">Tap Me</button>',
+				'<button on="tap:my-lightbox">Tap Me</button>'
+			),
+
 			'multiple_blacklisted_attributes' => array(
 				'<a href="/path/to/file.jpg" style="border: 1px solid red;" onclick="alert(e);">Link</a>',
 				'<a href="/path/to/file.jpg">Link</a>',

tests/test-amp-img-sanitizer.php

@@ -37,6 +37,11 @@ public function get_data() {
 				'<amp-img src="http://placehold.it/350x150" width="350" height="150" alt="Placeholder!" sizes="(min-width: 350px) 350px, 100vw" class="amp-wp-enforced-sizes"></amp-img>',
 			),
 
+			'image_with_on_attribute' => array(
+				'<img src="http://placehold.it/350x150" on="tap:my-lightbox" />',
+				'<amp-img src="http://placehold.it/350x150" on="tap:my-lightbox"></amp-img>',
+			),
+
 			'image_with_blacklisted_attribute' => array(
 				'<img src="http://placehold.it/350x150" style="border: 1px solid red;" />',
 				'<amp-img src="http://placehold.it/350x150"></amp-img>',