kunit: building kunit as a module breaks allmodconfig

kunit tests that do not support module build should depend
on KUNIT=y rather than just KUNIT in Kconfig, otherwise
they will trigger compilation errors for "make allmodconfig"
builds.

Fixes: 9fe124bf ("kunit: allow kunit to be loaded as a module")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>