Tests fail if ~/.esmvaltool/config-user.yml file exists

[bouweandela]

Since https://github.com/ESMValGroup/ESMValCore/pull/2448 was merged, several unit tests fail if the file ~/.esmvaltool/config-user.yml exists.

FAILED tests/unit/config/test_config_object.py::test_reload - AssertionError: assert Config({'auxi...tent': False}) == Config({'auxi...tent': False})
FAILED tests/unit/config/test_config_object.py::test_reload_fail - Failed: DID NOT RAISE <class 'esmvalcore.exceptions.InvalidConfigParameter'>
FAILED tests/unit/main/test_esmvaltool.py::test_header - AssertionError: assert 'Reading conf...[deprecated])' == 'Reading conf...ine argument)'
FAILED tests/integration/test_deprecated_config.py::test_no_deprecation_default_cfg - esmvalcore.exceptions.ESMValCoreDeprecationWarning: Usage of the single configuration file ~/.esmvaltool/config-user.yml or specifying it via CLI argument `--config_file` has been deprecated in ESMValCore version 2.12.0 and is...

@schlunma Would you have time to take a look at this?

[schlunma]

Will do! Looks like I never ran all tests with an old config file on my machine.