net: filter: improve test case framework

This patch simplifies and refactors the test case code a
bit and also adds a summary of all test that passed or
failed in the kernel log, so that it's easier to spot if
something has failed.

Future work could further extend the test framework to also
support different input 'stimuli' i.e. related structures
to seccomp.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>