test/test-downtime: Fixes to single-run code

Previously the code run when the --single-run option would only ever run a
flexible check. Now it will run the check requested and also display the
check requested.
downtime: Downtime initialized before retention data read

The downtime data needs to be initialized before the retention data is
read so when there was a downtime in effect when core was stopped it will
be recreated on restart.