Unittests for pkgmgr plugins management code

Unittests for pkgmgr plugins management code

Run new tests:
 $ /usr/bin/app-installers-ut/plugins_unittest

This test requires several helper libraries to test
if plugins are called in correct way. Commit implements
3 test plugins and test assessor library. Test assessor
library is used by plugins and test code to set and get
results of plugin execution.

Change-Id: Idc30b953d6e5453c0e8fde94e1eea6d691225ea3