ztest: Give error if user has not defined CONFIG_ZTEST

If user is trying to use ztest by including ztest.h but has
not defined CONFIG_ZTEST, then fail the compilation as the
result binary will not do anything.

There is no check for CONFIG_ZTEST for unit tests that are run
without qemu.

Change-Id: Ief9bba3a3a0f6acc6a264f17cde828b6d7e543a5
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>