REGTESTS: connection: test the expect-proxy action

REGTESTS: connection: test the expect-proxy action

"tcp-request connection expect-proxy layer4" had no coverage. Unlike
the "accept-proxy" bind option, which makes the PROXY protocol header
mandatory for everybody, this action only expects it from the
connections matching its condition.

The test announces addresses which differ from the real ones so that
src and dst show which of the two was used, checks that fc_src keeps
reporting the real peer of the connection, that a connection from a
matching source is rejected when the header is missing or malformed,
and that a source which does not match the condition is served without
any header.