[feature/avatars] Do not use gravatar avatar in tests

PHPBB3-10018

tests/avatar/driver/foobar.php

@@ -0,0 +1,19 @@
+<?php
+
+class phpbb_avatar_driver_foobar extends phpbb_avatar_driver implements phpbb_avatar_driver_interface
+{
+	public function get_data($row)
+	{
+		return array();
+	}
+
+	public function prepare_form($template, $row, &$error)
+	{
+		return false;
+	}
+
+	public function process_form($template, $row, &$error)
+	{
+		return false;
+	}
+}