Fix third party scheduled tests running on forks

[brianschubert]

Background in #226, #522

Moves a comment to outside of the if: condition so that the condition doesn't always evaluate to true (see #522 (comment)).

Also did some opportunistic refactoring:

• Simplified the condition: A || (B && ~A) is equivalent to A || B
• Refactored the repeated condition into a single predicate job. This way we don't have to repeat the same logic in each test job. I made this change in a separate commit which I can roll back if keeping the original setup is preferred.