CMakeLists.txt: fix googletest related options

[shr-project]

• https://cmake.org/cmake/help/book/mastering-cmake/chapter/Writing%20CMakeLists%20Files.html says that CMake options are case-sensitive and I don't see lower-case version in currently used googletest submodules and gtest is indeed installed with leveldb

• install_gmock option I don't see at all, so I've kept it as is, INSTALL_GTEST, BUILD_GMOCK do exist as upper-case

$ grep -Ri install_.*mock .
./CMakeLists.txt: set(install_gmock OFF)
./third_party/googletest/googlemock/CMakeLists.txt:install_project(gmock gmock_main) $ grep -Ri build_gmock .
./CMakeLists.txt: set(BUILD_GMOCK ON)
./third_party/googletest/googletest/README.md:cmake .. -DBUILD_GMOCK=OFF ./third_party/googletest/CMakeLists.txt:option(BUILD_GMOCK "Builds the googlemock subproject" ON) ./third_party/googletest/CMakeLists.txt:if(BUILD_GMOCK) $ grep -Ri install_gtest .
./CMakeLists.txt: set(INSTALL_GTEST OFF)
./third_party/googletest/googletest/cmake/internal_utils.cmake: if(INSTALL_GTEST) ./third_party/googletest/googletest/CMakeLists.txt:if (INSTALL_GTEST) ./third_party/googletest/CMakeLists.txt:option(INSTALL_GTEST "Enable installation of googletest. (Projects embedding googletest may want to turn this OFF.)" ON)

• also use CACHE and FORCE as sugested in: https://cmake.org/cmake/help/latest/command/set.html https://stackoverflow.com/questions/20239334/cmake-set-subdirectory-options for the value to correctly propagate into third_party/googletest subdirectory

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