oeqa/selftest: add oe-selftest for kernel module pre-filter

oeqa/selftest: add oe-selftest for kernel module pre-filter

Add selftest-ko-filter recipe and a PackageTests.test_kmodule_prefilter
oe-selftest that builds it to verify process_split_and_strip_files()
handles .ko.xz/.ko.gz alongside a real .ko without corrupting them.
Regression test for YOCTO #2348.

AI-Generated: Uses Claude Sonnet 4.6

Signed-off-by: Sam Kent <sam.john.kent@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>