## Summary

Closes elastic#162045

This PR adds an API integration test for the following scenario:
- Given an integration with some non secret (plain text) vars that
become secret in a newer version;
- When Fleet has an agent policy with this integration and upgrades from
the old to the newer version;
- Then the vars that have become secrets should correctly be stored as
secret values.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

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Co-authored-by: Julia Bardi <90178898+juliaElastic@users.noreply.github.com>
(cherry picked from commit 766ff8f)