Fix how a change to the parent course ID is checked in prepare_item_f…

[pondermatic]

Description

Fixed how a change to a lesson's parent course ID is checked. It was using an undefined variable.

Fixes #289.

How has this been tested?

Manually and with a new unit test.

Types of changes

Bugfix (non-breaking change which fixes an issue)

Checklist:

• [x] My code has been tested.
• [x] My code passes all existing automated tests.
• [x] My code follows the LifterLMS Coding & Documentation Standards.

[pondermatic]

Sorry @eri-trabiccolo, I didn't see that #284 also fixes issue #289. Feel free to close this pull request.

[eri-trabiccolo]

@pondermatic thanks for this, I'm closing this but I'm stealing the test. See b4719eb