Drop test dependency on should_be

Fixes: pythongssapi#215
Merges: pythongssapi#219

Signed-off-by: Stanislav Levin <[email protected]>
Signed-off-by: Robbie Harwood <[email protected]>

[[email protected]: bugfixes, rebase around six changes, not abusing
Python equating empty and False, misc additional test cleanups, commit
message]

Author: stanislavlevin