bpf: Add BPF_MAP_TYPE_SK_STORAGE test to test_maps

This patch adds BPF_MAP_TYPE_SK_STORAGE test to test_maps.
The src file is rather long, so it is put into another dir map_tests/
and compile like the current prog_tests/ does.  Other existing
tests in test_maps can also be re-factored into map_tests/ in the
future.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>