Revert "Test that `-takeUntil:` invokes disposal on subscribed-to signals"

This reverts commit 4d5ca8976344ffe10f83b8d6c99ed97d67814508.
Revert "Test disposal of `+merge:` signal invokes subscribed-to signals' disposal"

This reverts commit a3fa2a092f50656a46833cce5db324f562f703a4.
Test that disposables upstream of a completing signal are disposed