Resolve "statschannel test intermittently fails with incorrect zone loadtime"

The check_loaded() function compares the zone's loadtime value and an expected loadtime value, which is based on the zone file's mtime extracted from the filesystem.

For the secondary zones there may be cases, when the zone file isn't ready yet before the zone transfer is complete and the zone file is dumped to the disk, so a so zero value mtime is retrieved.

In such cases wait one second and retry until timeout. Also modify the affected check to allow a possible difference of the same amount of seconds as the chosen timeout value.

Closes #4477 (closed)