Tests are not idempotent

[TomBurdge]

Currently, the tests are not idempotent. In other words, sometimes they fail and sometimes they succeed.

The tests create directories/files with over-lapping names. I think this may be causing behaviour; the errors tend to be based on the order of the tests being run with cargo test, which is pseudo-random. This is slightly perplexing behaviour, because the tests appear to tear up/down everything themselves. I think the pattern for this, is to use tempfile or a similar crate, as currently the directories/files are created quite directly.

There might be a super straightforward explanation, in need of a small re-factor.

[maxwellflitton]

@TomBurdge thanks for this I'll run some local tests myself off the top of my head I think that we just set the thread for tests to 1 to avoid clashes? I guess building out some commands in cargo make would be a good shout I can build some test commands and also some CI on the tests