m68k: defconfig: Update defconfigs for v6.14-rc1

  - Drop CONFIG_DM_KUNIT_TEST=m (auto-modular since commit
    55b7aee9 ("drivers: base: test: Enable device model tests
    with KUNIT_ALL_TESTS")),
  - Re-enable modular build of MD Linear (append) mode (revived in
    commit 127186cf ("md: reintroduce md-linear").

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/be6ac546f7b98eae352584daacc42a58577dc224.1738589762.git.geert@linux-m68k.org