Simplify config tests by using T.Setenv, T.TempDir

[alexandear]

The PR simplifies config tests by refactoring code and without harming readability:

• Use T.Setenv instead of os.Setenv. Because T.Setenv calls os.Setenv(key, value) and uses T.Cleanup to restore the environment variable to its original value after the test. So, the test doesn't need to restore the environment by itself.
• Use T.TempDir instead of os.CreateTemp to create a temporary file. TempDir returns a temporary directory which is automatically removed when the test and all its subtests complete.

[jackc]

Thanks!