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


git-svn-id: https://develop.svn.wordpress.org/trunk@41730 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/plural-form-function.php

@@ -0,0 +1,16 @@
+<?php
+
+/**
+ * Legacy plural form function.
+ *
+ * @param int $nplurals
+ * @param string $expression
+ */
+function tests_make_plural_form_function( $nplurals, $expression ) {
+	$expression = str_replace( 'n', '$n', $expression );
+	$func_body = "
+		\$index = (int)($expression);
+		return (\$index < $nplurals)? \$index : $nplurals - 1;";
+
+	return create_function( '$n', $func_body );
+}