need tests for create-{client, node} with default nodedir

[davidsarah]

We shouldn't create a node at the actual default basedir (which might already exist), which is why there are no existing tests for this.

However, we could:

• check that the result of get_default_nodedir​ is sensible;
• mock get_default_nodedir to return a default nodedir under the _trial_temp directory.

To test the logic for getting the default nodedir from the registry entry on Windows, we can mock sys.platform and allmydata.windows.registry.get_registry_value.