Use macros for test and helper executable targets on cmake build system.

Use macros for test and helper executable targets on cmake build system.

The new macros add_test_executables and add helper_executables provides a
platform independent way for specifing dbus test and service applications.

On native Windows and Linux/UNIX systems the test applications are
directly runable.

When cross compiling for Windows on Linux test applications could be
executed on the Linux host system with the help of wine and activated
binfmt_misc support for wine.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=41252
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>