Replace the direct usages of `make_passes` with `get_passes`.

Created by: tomprince

The later function takes a count of passes to generated, and passes them to make_passes. This also changes make_passes to return Passes, which is what get_passes wants to receive.

As part of working on https://whetstone.privatestorage.io/privatestorage/PrivateStorageio/-/issues/84 I want access to the tokens that are generated by make_passes/get_passes; by using a function that returns Pass, I can avoid needing to parse the result to make assertions.