Reduce flakiness of atomic slot migration AOF test (#2705)

If we don't wait for the replica to resync, the migration may be
cancelled by the time the replica resyncs, resulting in a test failure
when we can't find the migration on the replica.

---------

Signed-off-by: Jacob Murphy <[email protected]>
Signed-off-by: Viktor Söderqvist <[email protected]>
Co-authored-by: Viktor Söderqvist <[email protected]>

Author: murphyjacob4

tests/unit/cluster/cluster-migrateslots.tcl

@@ -2174,8 +2174,9 @@ start_cluster 3 3 {tags {logreqres:skip external:skip cluster aofrw} overrides {
         wait_for_migration_field 3 $jobname state occurring-on-primary
         assert_match "0" [R 3 DBSIZE]
 
-        # Restart the replica
+        # Restart the replica and wait for resync
         do_node_restart 3
+        wait_for_sync [srv -3 client] 50 1000
 
         # Replica should still see the migration. Note that since AOF does not
         # persist the replication ID, this is because of a full resync.