selftests/bpf: Add bpf_core_field_offset() tests

Add test cases for bpf_core_field_offset() helper.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220509004148.1801791-7-andrii@kernel.org