bpf, selftests: Add option to test_sockmap to omit adding parser program

Add option to allow running without a parser program in place. To test
with ping/pong program use,

 # test_sockmap -t ping --txmsg_omit_skb_parser

this will send packets between two socket bouncing through a proxy
socket that does not use a parser program.

   (ping)                                    (pong)
   sender         proxy_recv proxy_send      recv
     |                |                       |
     |              verdict -----+            |
     |                |          |            |
     +----------------+          +------------+

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/160239300387.8495.11908295143121563076.stgit@john-Precision-5820-Tower