kunit: add a convenience allocation wrapper for SKBs

Add a simple convenience helper to allocate and zero fill an SKB for the
use by a kunit test. Also provide a way to free it again in case that
may be desirable.

This simply mirrors the kunit_kmalloc API.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://msgid.link/20231220151952.415232-3-benjamin@sipsolutions.net
[adjust file description as discussed]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>