[8.15] Wait between instances, but not after the last one (elastic#19…

…0679) (elastic#190694)

# Backport

This will backport the following commits from `main` to `8.15`:
- [Wait between instances, but not after the last one
(elastic#190679)](elastic#190679)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Alejandro Fernández
Haro","email":"[email protected]"},"sourceCommit":{"committedDate":"2024-08-19T12:46:15Z","message":"Wait
between instances, but not after the last one
(elastic#190679)","sha":"965b0a675d537f3a83dcf7eaa6153a33af1b62f4","branchLabelMapping":{"^v8.16.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["Team:Core","release_note:skip","backport:all-open","v8.16.0"],"title":"Wait
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(elastic#190679)","sha":"965b0a675d537f3a83dcf7eaa6153a33af1b62f4"}},"sourceBranch":"main","suggestedTargetBranches":[],"targetPullRequestStates":[{"branch":"main","label":"v8.16.0","branchLabelMappingKey":"^v8.16.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/190679","number":190679,"mergeCommit":{"message":"Wait
between instances, but not after the last one
(elastic#190679)","sha":"965b0a675d537f3a83dcf7eaa6153a33af1b62f4"}}]}]
BACKPORT-->

Co-authored-by: Alejandro Fernández Haro <[email protected]>

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);
       }
     }