tests: Don’t bother restoring HOME

Some unittests set environment variables, but then delete them as part
of their cleanup process. Deleting them is OK. Any test that needs an
environment variable should set that environment variable itself. Once
the test process stops, any changes made to the environment will be lost
[1].

Before this change, there was one location where an environment variable
was restored to its original value instead of deleted. Restoring that
variable was unnecessary.

This commit was created to prepare for a future commit which will delete
HOME before any of the tests even start. Without this change, that
future change would crash. You can’t restore a variable that’s been
deleted.

Fixes #605.

[1]: <https://stackoverflow.com/q/716011/7593853>

tests/test_cli.py

@@ -315,7 +315,7 @@ def test_run_with_user_global_config_file(self):
         os.makedirs(dir)
         config = os.path.join(dir, 'config')
 
-        self.addCleanup(os.environ.update, HOME=os.environ['HOME'])
+        self.addCleanup(os.environ.__delitem__, 'HOME')
         os.environ['HOME'] = home
 
         with open(config, 'w') as f: