selftests/bpf: Add BTF sanity tests

Preparing the metadata for bpf_list_head involves a complicated parsing
step and type resolution for the contained value. Ensure that corner
cases are tested against and invalid specifications in source are duly
rejected. Also include tests for incorrect ownership relationships in
the BTF.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118015614.2013203-24-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>