Eero Aaltonen authored Oct 17, 2021 and Brad King committed Nov 22, 2021

`googlemock` has been absorbed into the
[googletest](https://github.com/google/googletest) project and is built
and installed from the same source tree.

As GTest may be built with or without GMock, skip GMock if it is not
present.

Do not provide result variables for GMock.  They are not provided by
upstream GTest's CMake Package Configuration File.

Also update the test case to cover linking to `GTest::gmock`.