m68k: defconfig: Enable KUnit tests

Enable KUnit and all KUnit tests for modular builds, so they are
available when needed, just like non-KUnit tests.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20201026122622.3092658-1-geert@linux-m68k.org