Add a test for the C++ backend

beat/core/schema/common/1.json

...core/test/prefix/algorithms/user/cxx_integers_echo/1.json

...t/prefix/algorithms/user/cxx_integers_echo/CMakeLists.txt

...st/prefix/algorithms/user/cxx_integers_echo/algorithm.cpp

...test/prefix/algorithms/user/cxx_integers_echo/algorithm.h

...t/prefix/algorithms/user/cxx_integers_echo/beat_setup.cpp

...est/prefix/algorithms/user/cxx_integers_echo/beat_setup.h

...gorithms/user/cxx_integers_echo/user_single_integer_1.cpp

...algorithms/user/cxx_integers_echo/user_single_integer_1.h

...est/prefix/experiments/user/user/double/1/cxx_double.json

beat/core/test/test_docker.py

beat/core/test/test_execution.py

beat/core/utils.py

buildout.cfg