Fix a statschannel system test zone loadtime issue

Fix a statschannel system test zone loadtime issue

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.