REGTESTS: log: check that a log-forward TCP session ends on client close

REGTESTS: log: check that a log-forward TCP session ends on client close

This test covers the bug fixed by the previous commit:

    BUG/MEDIUM: log: do not ignore the peer's shutdown in the syslog applet

A log-forward section without "timeout client" accepts a single
connection at a time ("maxconn 1"). A first client sends a message and
only closes once the message was forwarded, so that its shutdown is
received alone, on an empty input buffer. A second client then sends
another message, which can only be forwarded once the first session is
gone. On an affected, unpatched build, the first session never ends and
the test fails on its timeout. It passes on 3.2, which is not affected.

This must be backported wherever the fix above is backported.