selftests/bpf: add tests for BPF object load with implicit token

selftests/bpf: add tests for BPF object load with implicit token

Add a test to validate libbpf's implicit BPF token creation from default
BPF FS location (/sys/fs/bpf). Also validate that disabling this
implicit BPF token creation works.

Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231213190842.3844987-9-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>