In #67, we found an out-of-date-mock was leading to a diff in the expected
output for the upgrade test. Updating the mock this trivial, but why did it
drift?

This PR makes some improvements to help debug this:

• Prevent implicit fixtures-recording on CI

  This will ensure that any change that introduces drift (assuming it goes via
  PR with CI) will be identified then and there. I assume the immediate feedback
  will make it simple to understand why and take appropriate action.

• Capture /arguments when fixtures are written

  When we do encounter the drift error, we can now see which calls led to which
  existing fixtures. Perhaps this will be useful for identifying cause.

• Remove stale fixtures

  Any fixtures without /arguments after the last complete run are no longer used
  by the test suite. They are just noise and so have been removed.

Unfortunately, this will only help the next case of this, since we're not able
to capture arguments from the stale fixtures to debug, nor were we enforcing no
implicit fixture updates on CI before this PR.

Therefore, I have to give up and actually just commit the original update,
without any explanation for it.