Add quic::test::GetFirstFlightOfPackets() This CL adds GetFirstFlightOfPackets, a mechanism to extract the first flight of packets from a QUIC connection. This will be used to test code that parses the CHLO. This CL also adds various test classes and methods that are used by GetFirstFlightOfPackets. gfe-relnote: n/a, test-only PiperOrigin-RevId: 307703986 Change-Id: Iedd17f41f1120d2be2188694710dcd698d61efc8