Skip KUnit tests in the configuration, not on the command line

[tamnd]

#61 skipped the flaky KUnit suites with kunit.filter=module!=... on the command line. On 7.2.8 that filter panics the kernel before any test runs: attr_string_filter calls strcmp on the module name of a test case that has none, so every M14 KUnit boot died and the cells landed at L6. I stopped both queues once the first cells showed it.

This moves the skip into the configuration step. The # gk:kunit-skip line now names Kconfig options. After the fragment is applied, gk turns KUNIT_ALL_TESTS off in the resolved .config, which shows the test prompts again with the values it gave them, turns the named options off and runs olddefconfig again into kunit-skip.log. I tried that by hand on a 7.2.8 cell's config: the only symbols that change are the two tests, KUNIT_ALL_TESTS and DRM_SCHED, which only the scheduler test selects. Every other test stays on. A named option that is still on afterwards shows up in fragment_missed.

The boot side of #61 is reverted. M14 skips DRM_SCHED_KUNIT_TEST and RATELIMIT_KUNIT_TEST, and spec 07 explains why the command line filter is not used.

Checked on server2: fmt, clippy with -D warnings, tests and the style script pass.