dns-test: spurious test failure tshark::tests::nameserver

[japaric]

failed CI job: https://github.com/ferrous-systems/dnssec-tests/actions/runs/8882352128/job/24386825138#step:4:1365

this assertion failed: wait_for_capture returned one DNS message instead of two.

Most likely, tshark printed "new packets 1" and then immediately printed "new packets 2"; wait_for_capture exits on the first line match so it didn't see the second stdout line.

I have not run into this issue locally and CI has run into it once out of dozens of run so, so far, this seems low probability to not worry about it just yet.