selftests/bpf: Add type match test against kernel's task_struct

This change extends the existing core_reloc/kernel test to include a
type match check of a local task_struct against the kernel's definition
-- which we assume to succeed.

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220628160127.607834-11-deso@posteo.net