Refactored test_build.py

[mdellweg]

Made use of more generic with[git]project decorators.

Description

Checklist

• [ ] Make sure changes are covered by existing or new tests.
• [ ] For at least one Python version, make sure local test run is green.
• [ ] Create a file in src/towncrier/newsfragments/. Describe your change and include important information. Your change will be included in the public release notes.
• [ ] Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• [ ] Ensure docs/tutorial.rst is still up-to-date.
• [ ] If you add new CLI arguments (or change the meaning of existing ones), make sure docs/cli.rst reflects those changes.
• [ ] If you add new configuration options (or change the meaning of existing ones), make sure docs/configuration.rst reflects those changes.

[adiroiban]

It looks good. I don't have time to look into the details... but I think that we are fine. It's better than we have in trunk, the tests pass, so it shoudl be good to merge... the bar is low, I know, but this is what we can do now.