bpf: sockmap, add a set of tests to run by default

If no options are passed to sockmap after this patch we run a set of
tests using various options and sendmsg/sendpage sizes. This replaces
the sockmap_test.sh script.
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>