`fs_config_default` unit test is flaky on CI

[t4lz]

Failed here: https://github.com/metalbear-co/mirrord/actions/runs/7466234067/job/20317188352?pr=2162#step:36:439

Successful here without any change in the code: https://github.com/metalbear-co/mirrord/actions/runs/7466234067/job/20317958535

[t4lz]

Also feature::fs::advanced::tests::advanced_fs_config_default

[Razz4780]

I think I remember doing a short investigation of this. The config unit tests may not be properly isolated - some tests are setting values in the current env and those values are later read by other tests when generating config.