REGTESTS: use two haproxy instances to distinguish the QUIC traces

REGTESTS: use two haproxy instances to distinguish the QUIC traces

The aim of this patch is to identify the QUIC traces between the QUIC frontend
and backend parts. Two haproxy instances are created. The c(1|2) http clients
connect to ha1 with TCP frontends and QUIC backends. ha2 embeds two QUIC listeners
with s1 as TCP backend. When the traces are activated, they are dumped to stderr.
Hopefully, they are prefixed by the haproxy instance name (h1 or h2). This is very
useful to identify the QUIC instances.