test(integration): pcapng end-to-end counts fail when many_interfaces.pcapng falls back to the synthesised stand-in #1478

tests/integration/test_pcapng_end_to_end.py fails whenever make_samples.py can't download the upstream many_interfaces.pcapng.

Seen on #1476, job 113845878658 (Integration Python 3.14), 2026-10-09:

• The download from raw.githubusercontent.com/wireshark/wireshark/master/test/captures/many_interfaces.pcapng failed with Connection reset by peer.
• The generator then printed that it was synthesising a stand-in and that the upstream capture was NOT used.
• Three assertions then failed, all expecting upstream frame counts:
  • 24 != 64 at :70
  • the Counter({'Frame': 24, …}) comparison at :88
  • 24 != 64 at :96
• The legs on the other four Pythons, and the same leg on fix(pcap,pcapng)!: a cut PCAP header or PCAP-NG block rebuilds as captured (#1470) #1475 and fix(sctp): rebuild a last chunk short of its padding as captured (#1474) #1477, passed. The PR's diff (ipv6.py, decorators.py) doesn't touch PCAP-NG.

Problem: the counts are pinned to the upstream file, but the generator substitutes a 24-frame stand-in without failing. So a network blip on the runner turns CI red.

Possible fixes:

• Skip these assertions when the fixture is the stand-in. The generator would need to record that, e.g. in a marker file.
• Or derive the expected counts from whichever fixture was written.
• Or make the stand-in match upstream's block counts.

The failed job on #1476 will be re-run once its workflow run finishes; it cannot be re-run while other legs are still going.