Wait for the test zone to finish re-loading

'rndc thaw' initiates asynchrous loading of all the zones
similar to 'rndc load'.  Wait for the test zone's load to
complete before testing that it is updatable again.

(cherry picked from commit 5b3238aa85)

bin/tests/system/rndc/tests.sh

@@ -86,6 +86,8 @@ status=$((status+ret))
 echo_i "rndc thaw"
 $RNDCCMD 10.53.0.2 thaw | sed 's/^/ns2 /' | cat_i
 
+wait_for_log 3 "zone_postload: zone nil/IN: done" ns2/named.run
+
 n=$((n+1))
 echo_i "checking zone now writable ($n)"
 ret=0