[VarDumper] fix tests under 5.3
minor #13097 [VarDumper] fix tests under 5.3 (nicolas-grekas)

This PR was merged into the 2.6 branch.

Discussion
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[VarDumper] fix tests under 5.3

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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333fb6f [VarDumper] fix tests under 5.3