build: simplify management of dependencies on object libraries in CMake `gtest` function

This commits changes the `gtest` function to `target_add_libraries`
library object targets `${lib}` directly instead of
`$<TARGET_OBJECTS:${lib}>`. As a consequence, the
`target_include_directories` from the object target `${lib}` are
also passed to the target to compile and including them in the caller of
`gtest` is not needed anymore.