tests: Don't use libafsauthent

Use liboafs_auth.la directly, rather than using libafsauthent, to
build the auth tests

Change-Id: Idf0e7a32626802b6113d21833ded09d9e20e3659
Reviewed-on: http://gerrit.openafs.org/9023
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>