Reduce number of files opened during testing

[jemrobinson]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
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🚦 Depends on

n/a

⤴️ Summary

Avoid opening a new log file for each test

🌂 Related issues

Closes #1950

🔬 Tests

Tested locally and in CI.

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
data_safe_haven
directories.py
data_safe_haven/logging
logger.py
plain_file_handler.py
Project Total

This report was generated by python-coverage-comment-action