Test suite: wait for ERROR message on QUIT

Test suite: wait for ERROR message on QUIT

Wait for the "ERROR :Closing connection" message sent by ngIRCd when
handling the QUIT command, do not wait for "Connection closed" which is
actually output by the telnet(1) command and is implementation
dependant! For example, on Haiku OS, this is not always(!) echoed (the
command seems to hang sometimes?) which results in unpredictable
failures in the test suite ...