fusefs: More accurately test the unique tokens in the test suite

Every fuse ticket has a "unique" token. As the name implies, they're
supposed to be unique. Previously the fusefs test suite verified their
uniqueness by relying on the fact that they are also sequential. But
they aren't guaranteed to be sequential. Enhance the tests by removing
that convenient assumption.

MFC after: 2 weeks
Sponsored by: Axcient