dispatcher: expose connection D-Bus path

dispatcher: expose connection D-Bus path

src/nm-dispatcher.c was passing the connection's D-Bus path to the
dispatcher, which was then ignoring it. Meanwhile,
test-dispatcher-envp was passing a fake D-Bus path with the wrong name
(which didn't actually matter since
nm_dispatcher_utils_construct_envp() ignored it anyway).

Fix it so that the path gets exposed as CONNECTION_DBUS_PATH, and
adjust the tests for that.