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

A test is added for arraymap and percpu arraymap. The test also
exercises the early return for the helper which does not
traverse all elements.
    $ ./test_progs -n 45
    #45/1 hash_map:OK
    #45/2 array_map:OK
    #45 for_each:OK
    Summary: 1/2 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/20210226204934.3885756-1-yhs@fb.com