selftests/bpf: add test for bpf_for_each_map_elem() with different maps

A test is added for bpf_for_each_map_elem() with either an arraymap or a
hashmap.
$ tools/testing/selftests/bpf/test_progs -t for_each
#93/1 for_each/hash_map:OK
#93/2 for_each/array_map:OK
#93/3 for_each/write_map_key:OK
#93/4 for_each/multi_maps:OK
#93 for_each:OK
Summary: 1/4 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Philo Lu <lulie@linux.alibaba.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240405025536.18113-4-lulie@linux.alibaba.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>