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.)
