maintenance: add test for GetLagTimes.php
Test the output of maintenance/GetLagTimes.php as well as the metrics
reported to statsd.
A future change can build upon this to enhance how a non replicating
database is reported which prints the misleading "0" and emit to statsd
a gauge at 0.
Bug: T358484
Change-Id: Id050f91d046c479161fdc0deb5f8b74865365118