add test covering loading admins from config

[janl]

No description provided.

[janl]

Travis suggests this new test is flakey.

Is config:set() async? e.g. can there be no admin configured if trying to config:get() right after?

[janl]

new patch with proper wait

[eiri]

This should do it, I think. For a reason why this needed - config:set is sync, but config change broadcasting and consequently pass hashing isn't, so half of time get_admin/1 in couch_auth cache will be getting "password" instead of prefixed hash and default to nil