Advertise need for 'python-dpkt' system module

It's needed by TCP-related tracing commands, as of now:

 trace.py list --tcpdump
 trace.py tcpdump
trace: fix recommendation given by error message in case dpkt is missing

Since we adveritse use of a system package in README, we should also
recommend the same here.
tests: introduce tracing smoke test

The test verifies that:
 * samples are collected by OSv
 * extraction via `trace extract` succeeds
 * `trace summary` shows expected tracepoint names
 * listing via `trace list` shows expected tracepoints
 * network packet parsing works

This test is run as part of standard 'make check'.

Note: this single test can be executed as easily as:

 $ scripts/test.py --test tracing_smoke_test