Fix tests if deprecated `~/.esmvaltool/config-user.yml` file is available

[schlunma]

Description

Fix tests if a deprecated ~/.esmvaltool/config-user.yml file exists on the system.

Closes https://github.com/ESMValGroup/ESMValCore/issues/2541

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.90%. Comparing base (5b28372) to head (a6884af). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2543 +/- ## ======================================= Coverage 94.90% 94.90% ======================================= Files 251 251 Lines 14295 14295 ======================================= Hits 13566 13566 Misses 729 729 ```

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[schlunma]

config-developer.yml has not been deprecated (yet), so no problems here.

[valeriupredoi]

danke, Manu :beer:

[valeriupredoi]

@bouweandela would you spare a spare rib, I mean minute for a check and merge here pls bud :beer: