REGTESTS: healthcheckmail: Relax matching on the healthcheck log message

REGTESTS: healthcheckmail: Relax matching on the healthcheck log message

Depending on the timing, the conneciton on lisrv listener may be fully
accepted before any reject. Thus, instead of getting a socket error, an
invalid L7 response is reported. There is no reason to be strick on the
error type. Any failure is good here, because we just want to test the
email-alert feature.

This patch should fix issue #1857. It may be backported as far as 2.2.