selftests/bpf: Add test for strict BTF type check

Ensure that the edge case where first member type was matched
successfully even if it didn't match BTF type of register is caught and
rejected by the verifier.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220424214901.2743946-14-memxor@gmail.com