cmake: link against gtest in a better way

cmake: link against gtest in a better way

* add FindGMock.cmake which allows user to use the libgtest-dev
  shipped by distro
* add GMock::{GMock,Main}, GTest::{GTest,Main} targets to be
  compatible with FindGTest.cmake and FindGMock.cmake, which
  expose the built libraries with properties adhered to
  them. so the consumer of them can import them in a better way.
* update tests to drop the commands like
  set_target_properties(foo PROPERTIES COMPILE_FLAGS ${UNITTEST_CXX_FLAGS}),
  as they are already linked against gmock and gtest.

Signed-off-by: Kefu Chai <kchai@redhat.com>