selftests/bpf: Add bpf_arena_htab test.

bpf_arena_htab.h - hash table implemented as bpf program

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240308010812.89848-15-alexei.starovoitov@gmail.com