selftests/bpf: Add test for bpf array map iterators

Two subtests are added.
  $ ./test_progs -n 4
  ...
  #4/20 bpf_array_map:OK
  #4/21 bpf_percpu_array_map:OK
  ...

The bpf_array_map subtest also tested bpf program
changing array element values and send key/value
to user space through bpf_seq_write() interface.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200723184121.591367-1-yhs@fb.com