The latest version of googletest provides several improvements that
require adaptation of the current unit test implementation for xplugin:
- fix deprecation warnings caused by terminology changes,
- apply a new version of the macro to create function mocks.
Moreover, to clean up previously neglected topics:
- move mock classes to separate namespace,
- update the breakdown of mocking classes with corresponding header
  files.

RB: 25460
Reviewed-by: Lukasz Kotula <lukasz.kotula@oracle.com>