Tests: Add tests for `wpdb::_real_escape()`.

Adds a new dedicated test file.

Adds a test to check that various input types passed to `wpdb::_real_escape()` are handled correctly.

Note: This new test does not test the actual escaping or other logic in the function. Rather, it just and only tests and documents how the function handles various input types.

Props jrf, hellofromTonya.
See #53363.
Built from https://develop.svn.wordpress.org/trunk@51798


git-svn-id: http://core.svn.wordpress.org/trunk@51405 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51797';
+$wp_version = '5.9-alpha-51798';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.