selftests/bpf: Add various tests to check helper access into ptr_to_btf_id.

Add various tests to check helper access into ptr_to_btf_id.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230313235845.61029-4-alexei.starovoitov@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>