selftests/bpf: Add BPF linked list API tests

Include various tests covering the success and failure cases. Also, run
the success cases at runtime to verify correctness of linked list
manipulation routines, in addition to ensuring successful verification.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118015614.2013203-23-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>