tools/bpf: add a testcase for MAP_GET_NEXT_KEY command of LPM_TRIE map

A test case is added in tools/testing/selftests/bpf/test_lpm_map.c
for MAP_GET_NEXT_KEY command. A four node trie, which
is described in kernel/bpf/lpm_trie.c, is built and the
MAP_GET_NEXT_KEY results are checked.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>