Giving students some of the test cases up-front with a pre-check but then deferring final marking to the end of the assessment is an interesting way to mark. We've never tried it. I assume you'd move to a partial marking system, rather than all-or-nothing, for such questions?
I was perhaps a little rash to suggest the Precheck message could also display a warning that final testing may fail. We can do that with our in-house question types because we use combinator template graders and can customise messages that come before or after the result table. Unless you're doing something similar, the best you'll be able to do is write a special Precheck test case that explicitly prints that message, which will appear in both the Expected and Got columns of the result table. Not pretty, but workable? Or (easier) just explain to everyone in the class how Prechecks work and what a Status of Precheck results means when they're making their final submission.