tests/auth: Refactor common code

tests/auth: Refactor common code

Pull the common code for creating a configuration directory out of
the keys and superuser tests into a single file. This both cleans up
the existing tests, and makes it easier to add new ones.

Change-Id: I08058117e08da3a3baf750b3b14ef6780f942206
Reviewed-on: http://gerrit.openafs.org/4049
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>