Editor: Add unit tests for v5.3.1 block serialization functions.

[46896] was intended to have included unit tests for the block serialization functions added as part of the changeset.

Props aduth.
Merges [46997] to the 5.3 branch.
Fixes #49048.
Built from https://develop.svn.wordpress.org/branches/5.3@47131


git-svn-id: http://core.svn.wordpress.org/branches/5.3@46931 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3.3-alpha-47130';
+$wp_version = '5.3.3-alpha-47131';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.