[SSSD] [PATCH v3] Add integration tests

[Nikolai Kondrashov]

Hi everyone,

Here is the third version of the integration test patches.

Changes from the second version include the following.

* Rebased on latest master.
* Moved verbs to the start of ent.py function names, so assertion function
   names start with "assert_".
* Hidden a few of the internal ent.py functions under the underscore prefix,
   removed others, please tell if anything else needs to be hidden.
* Made passwd/group dictionary matching use get(pw|gr)(nam|uid|gid) functions
   instead of get(gr|pw)ent, as originally requested for decoupling requested
   users/groups from patterns to match, for alias support. This concerns
   assert_(passwd|group)_(name|uid|gid)_dict functions.
* Added passing pattern structure descriptions to the diff function, so
   assertion failure messages provide more clue to what happened. Made some
   adjustments to message generation for the same purpose.
* Added support for matching only some entries. E.g. matching only a few users
   returned from getpwent, or matching only a few group members. Implemented
   with tuple patterns.
* Added "ent.contains" and "ent.contains_only" functions creating
tuple and
   list patterns to possibly help readability.

Thanks to anyone taking on the review of these patches!

Nick