bpf: selftest: Move sock_fields test into test_progs

This is a mechanical change to
1. move test_sock_fields.c to prog_tests/sock_fields.c
2. rename progs/test_sock_fields_kern.c to progs/test_sock_fields.c

Minimal change is made to the code itself.  Next patch will make
changes to use new ways of writing test, e.g. use skel and global
variables.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200925000427.3857814-1-kafai@fb.com