tests: unit: explicitly ask for dvc repo

[efiop]

Currently, these rely on the root of the repo being in the dvc repo.

Leftover from #9627

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.28 ⚠️

Comparison is base (92dc1f3) 90.52% compared to head (74a91a8) 90.25%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9628      +/-   ##
==========================================
- Coverage   90.52%   90.25%   -0.28%     
==========================================
  Files         471      471              
  Lines       36058    36058              
  Branches     5185     5185              
==========================================
- Hits        32643    32544      -99     
- Misses       2822     2897      +75     
- Partials      593      617      +24     

Impacted Files	Coverage Δ
tests/unit/command/test_add.py	100.00% <100.00%> (ø)
tests/unit/command/test_data_sync.py	100.00% <100.00%> (ø)
tests/unit/command/test_diff.py	100.00% <100.00%> (ø)
tests/unit/command/test_experiments.py	100.00% <100.00%> (ø)
tests/unit/command/test_imp.py	100.00% <100.00%> (ø)
tests/unit/command/test_imp_url.py	100.00% <100.00%> (ø)

... and 25 files with indirect coverage changes

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