selftests/bpf: add number iterator tests

Add number iterator (bpf_iter_num_{new,next,destroy}()) tests,
validating the correct handling of various corner and common cases
*at runtime*.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230308184121.1165081-8-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>