fusefs: fix some minor Coverity CIDs in the tests

Where open(2) is expected to fail, the tests should assert or expect that
its return value is -1. These tests all accepted too much but happened to
pass anyway.

Reported by: Coverity
Coverity CID: 1404512, 1404378, 1404504, 1404483
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation