Media: After [42693], update HTML4 markup in `img_caption_shortcode()` to avoid an empty ID attribute as well.

Add a unit test for `aria-describedby` in `img_caption_shortcode()`.

See #34595.
Built from https://develop.svn.wordpress.org/trunk@42694


git-svn-id: http://core.svn.wordpress.org/trunk@42522 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/media.php

@@ -1623,7 +1623,7 @@ function img_caption_shortcode( $attr, $content = null ) {
 			esc_attr( $class ),
 			str_replace( '<img ', '<img ' . $describedby, do_shortcode( $content ) ),
 			sprintf(
-				'<p id="%s" class="wp-caption-text">%s</p>',
+				'<p %sclass="wp-caption-text">%s</p>',
 				$atts['caption_id'],
 				$atts['caption']
 			)

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-42693';
+$wp_version = '5.0-alpha-42694';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.