Address test timing false positive in rndc system test.

Job #1931097 failed for fa52a4c8:

8738I:rndc:checking update (68)
8739I:rndc:rndc freeze
8740I:rndc:edit zone files
8741I:rndc:rndc thaw
8742I:rndc:rndc reload
8743I:rndc:ns7 server reload successful
8744I:rndc:checking zone file edits are loaded (69)
8745I:rndc:failed

The test digs where executed before the zone loading was complete.