Wait between instances, but not after the last one (#190679)

(cherry picked from commit 965b0a6)

src/core/server/integration_tests/saved_objects/migrations/group2/multiple_kibana_nodes.test.ts

@@ -183,7 +183,8 @@ describe('migration v2', () => {
           errors.push(err.message);
         })
       );
-      if (i < instances.length - 1) {
+      if (i < instances.length - 2) {
+        // We wait between instances, but not after the last one
         await delay(delayInSec * 1000);
       }
     }