Fix bug where the problemresolve can put a pkg into a heisenstate

Fix bug where the problemresolve can put a pkg into a heisenstate

The problemresolver will set the candidate version for pkg P back
to the current version if it encounters an impossible to satisfy
critical dependency on P. However it did not set the State of
the package back as well which lead to a situation where P is
neither in Keep,Install,Upgrade,Delete state.

Note that this can not be tested via the traditional sh based
framework. I added a python-apt based test for this.

LP: #1550741

[jak@debian.org: Make the test not fail if apt_pkg cannot be
 imported]