Add clearing text test

Author: sirreal

tests/phpunit/tests/html-api/wpHtmlProcessorModifiableText.php

@@ -181,6 +181,14 @@ public static function data_modifiable_text_special_leading_whitespace() {
 				"\r\nCR-N",
 				"<{$tag_name}>\n\nCR-N<!--x--></{$tag_name}>",
 			);
+
+			yield "{$tag_label} clear text" => array(
+				"<{$tag_name}>REPLACEME<!--x--></{$tag_name}>",
+				1,
+				'REPLACEME',
+				'',
+				"<{$tag_name}><!--x--></{$tag_name}>",
+			);
 		}
 	}
 }