selftests/bpf: Add hashmap test for bpf_for_each_map_elem() helper

A test case is added for hashmap and percpu hashmap. The test
also exercises nested bpf_for_each_map_elem() calls like
    bpf_prog:
      bpf_for_each_map_elem(func1)
    func1:
      bpf_for_each_map_elem(func2)
    func2:

  $ ./test_progs -n 45
  #45/1 hash_map:OK
  #45 for_each:OK
  Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210226204933.3885657-1-yhs@fb.com