Increase test coverage of the fields module

[vbmendes]

Change Summary

This PR adds test coverage to two uncovered test scenarios:

• Computed fields with the deprecated attribute set to True
• Computed fields with the deprecated attribute set to a str instance.

Related issue number

Contributes to #7656

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #9470 will not alter performance

_{Comparing vbmendes:increase_fields_test_coverage (4101675) with main (9030415)}

Summary

✅ 13 untouched benchmarks

[sydney-runkle]

Nice work!