This patch introduces a new cmake macro add_session_test_executable,
which uses dbus-run-session to start a dbus-daemon process with a
temporary session bus in the background and the desired client file.
add_session_test_executable requires additional environment variables
defined in the top level CMakeLists.txt.
Bug: https://gitlab.freedesktop.org/dbus/dbus/issues/135
Signed-off-by: Ralf Habacker <ralf.habacker@freenet.de>