tests: kernel/context support other posix boards

Make it possible to run in other posix boards.
By default, if the POSIX board does not define the TICK_IRQ
just run without that part of the test, printing a note.
The place where other POSIX boards should define it, is also
clear, and should be easy to keep those lines free from merge
conflicts in the future.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>