selftests: Add tests for automatic map pinning

This adds a new BPF selftest to exercise the new automatic map pinning
code.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/157269298209.394725.15420085139296213182.stgit@toke.dk