selftests/run_kselftest.sh: Add `--skip` argument option

selftests/run_kselftest.sh: Add `--skip` argument option

Currently the only way of excluding certain tests from a collection is by
passing all the other tests explicitly via `--test`. Therefore, if the user
wants to skip a single test the resulting command line might be too big,
depending on the collection. Add an option `--skip` that takes care of
that.

Link: https://lore.kernel.org/r/20260116-selftests-add_skip_opt-v1-1-ab54afaae81b@suse.com
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>