FindGTest: Add GTest::{gtest,gtest_main} library names
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 (closed)
Topic-rename: FindGTest-upstream-lib-names
Edited by Brad King