(PUP-3030) confine uniquefile unlink tests

Two of the new tests re: Uniquefile are not compatible with
windows.  (In the ruby source tree, they are skipped on windows.)

This commit confines them to only run on unix.
Merge pull request #2993 from cprice404/bug/master/PUP-3030-confine-uniquefile-unlink-tests-to-unix

(PUP-3030) confine uniquefile unlink tests