Tests: Silence Requests deprecation

[swissspidy]

At the moment we don't hide PHP warnings and deprecations in Behat tests anymore, which helps surface potential issues with the tess.

There is currently 1 test failing because of that.

Example test run: https://github.com/wp-cli/core-command/actions/runs/11136463362/job/30948399200#step:11:36

001 Scenario: Make sure files are cleaned up with mixed case # features/core-update.feature:264
      Then STDOUT should be a number                         # features/core-update.feature:341
        Actual value: 'Deprecated: The PSR-0 `Requests_...` class names in the Request library are deprecated. Switch to the PSR-4 `WpOrg\\Requests\\...` class names at your earliest convenience. in /tmp/wp-cli-test-run--66fcaeaf29b076.48217376/wp-includes/class-requests.php on line 24
        4' (Exception)

This fix should address this warning.

[swissspidy]

Okay that didn't work 🤔

[swissspidy]

Not sure how I could silence the deprecations if not using an mu-plugin...