A lot of JS tests do this kind of check:
I just found this test test (which I reviewed!) that doesn't include the "is the 'storageEngine' falsy" check. The result is that when you run that test in the core suite, it will be skipped.
I think we should just make the default value for 'storageEngine' be 'wiredTiger' and then we won't need to worry about tests being skipped erroneously.