Error on warnings raised in tests

[dstansby]

This PR catches any warnings that are emitted by tests, and errors on them. This is generally good practice, to make sure the code isn't emitting any warnings, or if it is that they are intended.

There were also some warnings I had to fix. Hopefully the fixes look right!

[codecov]

Codecov Report

Base: 85.26% // Head: 85.26% // No change to project coverage 👍

Coverage data is based on head (c346670) compared to base (4aaadba).
Patch has no changes to coverable lines.

❗ Current head c346670 differs from pull request most recent head 301e70a. Consider uploading reports for the commit 301e70a to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #140   +/-   ##
=======================================
  Coverage   85.26%   85.26%           
=======================================
  Files          11       11           
  Lines         536      536           
=======================================
  Hits          457      457           
  Misses         79       79           

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