Change testing/* to use single quotes

[tonyandrewmeyer]

To be consistent with the rest of the code, we should change all of testing/**/*.py to use ' in preference to ", as we did with the other code earlier in the year.

The testing/pyproject.toml configuration should be altered to remove the special case, then ruff can be run, and then we'll want to add the commit to the blame ignore list.

[dimaqq]

bike shed time: let's take lead from scneario and switch to double quotes 🙄