Wait for the role change and fix the timing issue in the new test

The test might be fast enough and then there is no change in the role
causing the test to fail. Adding a wait to avoid the timing issue:
```
*** [err]: valkey-cli make source node ignores NOREPLICAS error when doing the last CLUSTER SETSLOT
Expected '{127.0.0.1 23154 267}' to be equal to '' (context: type eval line 24 cmd {assert_equal [lindex [R 3 role] 2] {}} proc ::test)
```

Signed-off-by: Binbin <[email protected]>

tests/unit/cluster/replica-migration.tcl

@@ -164,6 +164,7 @@ start_cluster 4 4 {tags {external:skip cluster} overrides {cluster-node-timeout
         R 7 config set cluster-allow-replica-migration yes
 
         # Record the current primary node, server 7 will be migrated later.
+        set old_role_response [R 7 role]
         set old_primary_ip [lindex [R 7 role] 1]
         set old_primary_port [lindex [R 7 role] 2]
 
@@ -177,6 +178,15 @@ start_cluster 4 4 {tags {external:skip cluster} overrides {cluster-node-timeout
             fail "valkey-cli --cluster rebalance returns non-zero exit code, output below:\n$result"
         }
 
+        # Wait for server 7 role response to change.
+        wait_for_condition 1000 50 {
+            [R 7 role] ne $old_role_response
+        } else {
+            puts "R 3 role: [R 3 role]"
+            puts "R 7 role: [R 7 role]"
+            fail "Server 7 role response has not changed"
+        }
+
         wait_for_cluster_propagation
         wait_for_cluster_state "ok"