Configure flake8 to run on test code as well

[jwodder]

There have been several times today and/or yesterday when linting of test files would have saved me some failed trips through the test suite.

[codecov]

Codecov Report

Merging #187 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #187      +/-   ##
==========================================
- Coverage   76.56%   76.56%   -0.01%     
==========================================
  Files          41       41              
  Lines        3124     3123       -1     
==========================================
- Hits         2392     2391       -1     
  Misses        732      732              

Flag	Coverage Δ
#unittests	76.56% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
dandi/support/tests/test_digests.py	100.00% <ø> (ø)
dandi/tests/fixtures.py	97.36% <ø> (ø)
dandi/tests/skip.py	88.88% <ø> (ø)
dandi/tests/test_download.py	100.00% <ø> (ø)
dandi/tests/test_utils.py	100.00% <ø> (ø)
dandi/conftest.py	100.00% <100.00%> (ø)
dandi/support/tests/test_cache.py	93.00% <100.00%> (ø)
dandi/tests/test_organize.py	100.00% <100.00%> (ø)

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