Use context managers for file writes in tests

[kbaikov]

This removes the need to explicitly close files and avoids accidental unclosed files

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📚 Documentation preview 📚: https://sqlite-utils--618.org.readthedocs.build/en/618/

[simonw]

Thanks, this is neater.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.69%. Comparing base (17eb818) to head (2fc480a). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #618 +/- ## ======================================= Coverage 95.69% 95.69% ======================================= Files 8 8 Lines 2854 2854 ======================================= Hits 2731 2731 Misses 123 123 ```

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