Fix GMock library target usage
Fixes the following errors:
CMake Error at tests/adapter-tests/CMakeLists.txt:113 (add_dependencies):
The dependency target "GMock" of target "repowerd-adapter-tests" does not
exist.
CMake Error at tests/core-tests/CMakeLists.txt:71 (add_dependencies):
The dependency target "GMock" of target "repowerd-core-tests" does not
exist.