Use source_playfield.add_ball for ball_locks in multiball

[cobra18t]

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[sonarcloud]

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[jabdoa2]

Good find. Thanks!

[cobra18t]

This PR is causing an error in the MPF Docs test...

mpf test game_logic/multiballs/multiball_with_traditional_ball_lock.rst 
Parsing documentation page
.F.
======================================================================
FAIL: test_config_parsing (mpf.tests.MpfDocTestCase.MpfDocTestCase)
Block 1 (starting at 0)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/mpf/tests/MpfDocTestCase.py", line 216, in test_config_parsing
    method(*parts)
AssertionError: KABOOM! We just added a ball to lockdevice which has a capacity of 2 but already had 2 ball(s)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/mpf/tests/MpfDocTestCase.py", line 218, in test_config_parsing
    raise AssertionError("Error in assertion {} (Num: {}) of your tests.".format(command, line_no)) from e
AssertionError: Error in assertion add_ball_to_device (Num: 23) of your tests.

----------------------------------------------------------------------
Ran 3 tests in 0.253s

FAILED (failures=1)

[cobra18t]

Not sure why that Docs test is failing. self.source_playfield.add_ball() does not seem to do anything in that test so the lock device is not unloading its balls. Maybe source_playfield is not properly defined in the test?

In an actual game that does this same sort of multiball, it works fine. I am not sure what is unique about this test versus a regular game. There are also similar test in the normal test_Multiball.py unit tests that pass without issue.