selftests/bpf: Add test for bpf hash map iterators

Two subtests are added.
  $ ./test_progs -n 4
  ...
  #4/18 bpf_hash_map:OK
  #4/19 bpf_percpu_hash_map:OK
  ...

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200723184120.590916-1-yhs@fb.com