Using installed testtools

For basically the first time ever I'm now in a situation where I want to build a single module (MYMODULE) against a whole tree of dune modules that I've previously installed (into /usr/local). MYMODULE uses testtools for parametrization. Its configure now fails because dune-testtools/cmake/modules/DuneSystemtests.cmake wants to

dune_execute_process(COMMAND ${CMAKE_BINARY_DIR}/run-in-dune-env dune_extract_static.py
--ini ${STATVAR_INIFILE}
--file ${CMAKE_CURRENT_BINARY_DIR}/interface.log
 WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
ERROR_MESSAGE "Error extracting static info from ${STATVAR_INIFILE}")

which fails since dune_extract_static.py is not in the virtual environment. It's also nowhere in /usr/local. Not using the virtual environment is also not an option, since testtools requires DUNE_PYTHON_VIRTUALENV_SETUP=True, right?