cmake: FindGTest: Add target for gmock library

`googlemock` has been absorbed into the
[googletest](https://github.com/google/googletest

) project and is built
and installed from the same source tree.

`googletest` has provided a CMake Config-file Package starting with
GTest 1.8.1. `find_package(GTest ...)` by default dispatches first to
CMake Find Module. Starting with CMake commit
2327b4330cce157d616ff8b611b3e77568d00351 in CMake v3.20.0 the module
dispatches onward to the Config-file Package so that the same targets
are available. In pre v3.20.0 versions of CMake however the Find Module
masks the targets provided by the upstream `GTest` package.

Update `Modules/FindGTest.cmake` to provide the same targets as the
CMake Config-file Package and backwards compatible targets and result
variables.

Signed-off-by: Eero Aaltonen <eero.aaltonen@vaisala.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>