Make pytest output more verbose

[candleindark]

This PR makes pytest output more verbose so that failure of comparison between model objects and their expected output can be meaningfully depicted. This PR and dandi/dandi-schema#341 together form a solution to address the issue described at #1716 (comment).

[Copilot]

Pull Request Overview

This PR increases pytest verbosity to provide more detailed output when tests fail, particularly for model object comparisons. The change enables better debugging by showing more information about test failures.

Key Changes

• Increased pytest verbosity from -v to -vv in the tox configuration

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.08%. Comparing base (a79fffc) to head (35b1e40).
⚠️ Report is 86 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1729      +/-   ##
==========================================
- Coverage   75.11%   75.08%   -0.04%     
==========================================
  Files          84       84              
  Lines       11856    11856              
==========================================
- Hits         8906     8902       -4     
- Misses       2950     2954       +4     

Flag	Coverage Δ
unittests	75.08% <ø> (-0.04%)	⬇️

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[yarikoptic]

I think we should not do it by default! there is posargs so users can add -v when needed