REGTESTS: provide a reverse-server test

REGTESTS: provide a reverse-server test

Test support for reverse server. This can be test without the opposite
haproxy reversal support though a combination of VTC clients used to
emit HTTP/2 responses after connection.

This test ensures that first we get a 503 when connecting on a reverse
server with no idle connection. Then a dummy VTC client is connected to
act as as server. It is then expected that the same request is achieved
with a 200 this time.