add some more credits mode tests

to prevent problems such as:
https://missionpinball.com/forum/topic/0-30-from-0-21-migrator-error/#post-8897

mpf/tests/machine_files/credits/config/config_freeplay.yaml

@@ -0,0 +1,49 @@
+#config_version=4
+
+modes:
+    - credits
+
+machine:
+    min_balls: 0
+
+switches:
+    s_left_coin:
+        number:
+    s_center_coin:
+        number:
+    s_right_coin:
+        number:
+    s_esc:
+        number:
+    s_start:
+        number:
+        tags: start
+
+coils:
+    c_eject:
+        number:
+
+credits:
+  max_credits: 12
+  free_play: yes
+  service_credits_switch: s_esc
+  switches:
+    - switch: s_left_coin
+      type: money
+      value: .25
+    - switch: s_center_coin
+      type: money
+      value: .25
+    - switch: s_right_coin
+      type: money
+      value: 1
+  pricing_tiers:
+    - price: .50
+      credits: 1
+    - price: 2
+      credits: 5
+  fractional_credit_expiration_time: 15m
+  credit_expiration_time: 2h
+  persist_credits_while_off_time: 1h
+  free_play_string: FREE PLAY
+  credits_string: CREDITS

mpf/tests/test_CreditsMode.py

@@ -5,7 +5,10 @@
 class TestCreditsMode(MpfTestCase):
 
     def getConfigFile(self):
-        return 'config.yaml'
+         if self._testMethodName == "test_free_play_at_start":
+            return 'config_freeplay.yaml'
+         else:
+            return 'config.yaml'
 
     def getMachinePath(self):
         return 'tests/machine_files/credits/'
@@ -23,6 +26,20 @@ def start_game(self, should_work):
         else:
             self.assertIsNone(self.machine.game)
 
+    def start_two_player_game(self):
+        # game start should work
+        self.machine.playfield.add_ball = MagicMock()
+        self.machine.ball_controller.num_balls_known = 3
+        self.hit_and_release_switch("s_start")
+        self.advance_time_and_run()
+        self.assertIsNotNone(self.machine.game)
+        self.assertEqual(1, self.machine.game.num_players)
+
+        # add another player
+        self.hit_and_release_switch("s_start")
+        self.advance_time_and_run(1)
+        self.assertEqual(2, self.machine.game.num_players)
+
     def stop_game(self):
         # stop game
         self.assertIsNotNone(self.machine.game)
@@ -42,8 +59,12 @@ def testFreePlay(self):
 
         self.assertEqual("FREEEE", self.machine.get_machine_var('credits_string'))
 
-        # game start should work
-        self.start_game(True)
+        self.start_two_player_game()
+
+    def test_free_play_at_start(self):
+        self.assertEqual("FREE PLAY", self.machine.get_machine_var('credits_string'))
+
+        self.start_two_player_game()
 
     def testToggleEvents(self):
         self.assertTrue(self.machine.mode_controller.is_active('credits'))
@@ -60,7 +81,7 @@ def testToggleEvents(self):
         self.post_event("toggle_credit_play")
         self.assertEqual("FREE PLAY", self.machine.get_machine_var('credits_string'))
 
-        self.start_game(True)
+        self.start_two_player_game()
         self.stop_game()
 
         self.post_event("enable_credit_play")