`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`.
This introduces 2 new INTERFACE IMPORTED libraries: GTest::gtest and
GTest::gtest_main. They link to GTest::GTest and GTest::Main targets
respectively, therefore working as aliases. These new names map the
names of the targets from upstream GTest's CMake package config.
Fixes: #20255
Add a reference to one of Google Test's command-line flags to the
FindGTest test. This will ensure that we are using the correct compile
definitions on Windows, as the test will otherwise fail to link. (IOW,
this tests the changes made by the previous commit.)
