[Serializer] Fix XmlFileLoader test
minor #13108 [Serializer] Fix XmlFileLoader test (dunglas)

This PR was merged into the 2.7 branch.

Discussion
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[Serializer] Fix XmlFileLoader test

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | not yet

Fix a bug in `XmlFileLoaderTest` tests.

Commits
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efd39cf [Serializer] Fix XmlFileLoader test