Tests: Require `imagejpeg()` function in `WP_Widget_Media_Image::render_media()` test.

This outputs a proper message if the requirement is not met, instead of an obscure failure further in the test.

This test relies on multiple resized copies of a test JPEG image being generated and available.

Follow-up to [49010], [49024], [49025], [49045], [49050].

See #50639, #50640.
Built from https://develop.svn.wordpress.org/trunk@49052


git-svn-id: http://core.svn.wordpress.org/trunk@48814 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-alpha-49051';
+$wp_version = '5.6-alpha-49052';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.