selftests/net: packetdrill: add minimal client and server tests

selftests/net: packetdrill: add minimal client and server tests

Introduce minimal tests. These can serve as simple illustrative
examples, and as templates when writing new tests.

When adding new cases, it can be easier to extend an existing base
test rather than start from scratch. The existing tests all focus on
real, often non-trivial, features. It is not obvious which to take as
starting point, and arguably none really qualify.

Add two tests
- the client test performs the active open and initial close
- the server test implements the passive open and final close

Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260105172529.3514786-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>