I18N: Improvements to the tests for plural forms.

* Move the `create_function()` code into a file that's only loaded, and into a test that's only run, on PHP <= 7.2 to avoid deprecated warnings in 7.2+.
* Convert the test skipping into a failure if the GlotPress locale file cannot be downloaded.
* Ensure `test_exceptions` fails if an exception is not thrown.
* Docs improvements

See #41562, #40109

Built from https://develop.svn.wordpress.org/trunk@41730


git-svn-id: http://core.svn.wordpress.org/trunk@41564 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-41729';
+$wp_version = '4.9-alpha-41730';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.