Fix unit test

The unit tests incorrectly check that the server hello packets
with PQC is 1. The reason for is that the second packet is encrypted.

But if splitting the packets differently, it could be that both
server hello packet can be identified.
I.e. this change makes the test case resilient to changes in dtls packetization...that might come in a future version.

Bug: webrtc:367395350
Change-Id: Ie1719b2d182190a9bbe7c24ddb2b6964fad31707
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/443280
Auto-Submit: Jonas Oreland <jonaso@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46721}