make test data provider method static

Author: Sasha Anastasi

tests/admin_setting_managemfa_test.php

@@ -37,7 +37,7 @@ public function test_get_factor_combinations_default() {
         $this->assertEquals(0, count($combinations));
     }
 
-    public function test_get_factor_combinations_provider() {
+    public static function test_get_factor_combinations_provider() {
         $provider = [];
 
         $factors = [];