Gazebo Classic provides the gazebo::ServerFixture class in the gazebo/test/ServerFixture.hh header, to simplify developing tests on top of its libraries.
Unfortunatly, to use it you must use exactly the same version of googletest that Gazebo vendors in its gazebo/test/gtest directory. This repo contains exactly a copy of that directory, in a standalone git repo to simplify its inclusion in other repos via CMake's FetchContent module.