Make unit tests compatible to PHP <7.3

Author: schmengler

tests/src/FakeTagRepository.php

@@ -40,4 +40,18 @@ public function getLastUsedId(): int
         return array_key_last($this->tags);
     }
 
+}
+
+/*
+ * PHP 7.2 Polyfill
+ */
+if (!\function_exists('array_key_last')) {
+    function array_key_last(array $array)
+    {
+        if (empty($array)) {
+            return null;
+        }
+
+        return key(array_slice($array, -1, 1, true));
+    }
 }