Add some regression tests on generated package files

[dstansby]

Is Your Feature Request Related to a Problem? Please Describe

We don't currently have any regression tests for the files that are generated by the package template. As an example of where this would have been useful, see https://github.com/UCL-ARC/python-tooling/pull/309. I made a mistake that broke a couple of the pyproject.toml fields, that would have been more easily spotted if there was a test that compared pyproject.toml against a reference/expected file.

Describe the Solution You'd Like

Add a test that compares pyproject.toml to an expected/reference file, which fails if the generated file doesn't match. I would suggest starting with just pyprojec.toml to pin down the testing machinery, and then rolling out to other files in a follow up PR.

Describe Alternatives You've Considered

No response

Additional Context

No response