SymlinkValidatorTest doesn't test PreApplyEvent

Problem/Motivation

Right now, SymlinkValidatorTest covers status checks and PreCreateEvent, but not PreApplyEvent.

There's a technical reason for this: namely, it is very tricky to change the behavior of the mocked precondition after the stage has been created, but before it's been applied. It could be done, of course, but would take significant refactoring of the test, and possibly the validator too.

This is not a big deal, because the validator behaves exactly the same way on PreCreateEvent and PreApplyEvent (and StatusCheckEvent, for that matter), and we are testing PreCreateEvent, so we can be pretty confident that PreApplyEvent works fine too. But it would still be nice to explicitly cover it. Therefore, this issue exists, but is a minor thing.