Alaways show triggered deprecation warnings on the unit test.
Test the deprecated `runas` argument to `salt.modules.pip`.
`user` and `runas` now raise an exception. "There can be only one".

* Always show the deprecation message `runas` is not `None`.
* If both `user` and `runas` are used, that's an error! Full Stop!
* If `runas` is being used and `user` is not, adapt the code and don't fail.
Also deprecate `runas` on `salt.states.pip`.

* Added mock tests to check if the deprecation is triggered, and to see if the deprecation is also returned to the user in a warnings key.
Added test case to see if the usage of `user` and `runas` at the same time raises an exception.
Merge pull request #6345 from s0undt3ch/hotfix/pip-tests

PIP module and state test cases, and minor improvments