cmake: test_sysbuild() function

Introduce a `test_sysbuild()` function.

This function is intended to be used by samples that are dependent on
sysbuild. This function allows such samples to test if sysbuild was used
in the build process, and when sysbuild is not used, then print a
warning to the user, or even fail the build.

This is useful for samples that have two parts to function properly, for
example samples that needs to be build and flash on two or more cores.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>