It'd be nice to have an equivalent to g_test_dbus_add_service_dir to test service activation

Gio's GTestDBus helper class includes an add_service_dir() method that adjusts the bus configuration to add a directory for extra .service files:

https://developer.gnome.org/gio/stable/GTestDBus.html#g-test-dbus-add-service-dir

The normal pattern in using this is to construct a directory of .service files as part of the build (e.g. using CMake's configure_file()) that point at the executables in the build directory. So when making calls to those services on the testing bus, the not-yet-installed version of the service can be activated the same way it would be in production.