s390/Kconfig: Make modules sanity test a module-only option
The modules sanity test must be built as a module to actually exercise
module loading. Require KUNIT && m to prevent built-in builds and avoid
misuse, so it is either 'n' or 'm'.
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>