tests/pf/nat: endpoint_independent: wait for tcpdump to start

[siva]

Summary

This matches pflog.sh tests and fixes flaky tests found in CI[0].

[0] https://ci.freebsd.org/job/FreeBSD-main-amd64-test/28126/testReport/junit/sys.netpfil.pf/nat/endpoint_independent_exhaust/

PR: 289628

[glebius]

I already went through this in other test cases. A sleep for a second makes test slower to execute but still doesn't give a 100% protection against flakyness. When tests are executed in parallel on a slow machine you may still have a failure sometimes. Here is the solution I came to so far, poll until tcpdump(1)s are all blocked on "bpf" wait channel. See 38edf96b1787ce3d8c00e4466348dab891c7a9ea.

[siva]

Thanks for the feedback, this approach looks good. I've tested it with 2000+ runs,
the flakiness is gone.