Add tests for other ru plural rules.

Refs #9554

tests/TestCase/I18n/PluralRulesTest.php

@@ -53,6 +53,10 @@ class PluralRulesTest extends TestCase
             ['ru', 0, 2],
             ['ru', 1, 0],
             ['ru', 2, 1],
+            ['ru', 21, 0],
+            ['ru', 22, 1],
+            ['ru', 5, 2],
+            ['ru', 7, 2],
             ['sk', 0, 2],
             ['sk', 1, 0],
             ['sk', 2, 1],