selftests/bpf: Add a test for bpf sk_storage_map iterator

Added one test for bpf sk_storage_map_iterator.
  $ ./test_progs -n 4
  ...
  #4/22 bpf_sk_storage_map:OK
  ...

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200723184122.591591-1-yhs@fb.com