BF(TST): account for git-annex starting to "singularize" copy in drop

[yarikoptic]

test runs of datalad/git-annex started to fail, so I spotted it.

Introduced in
8.20210330-245-g4edde9870

I do not think it is worth tracking copy vs copies as only used in
the tests.

[codecov]

Codecov Report

Merging #5609 (4b05df7) into maint (b7eaa4b) will decrease coverage by 5.71%.
The diff coverage is 100.00%.

❗ Current head 4b05df7 differs from pull request most recent head 786a44d. Consider uploading reports for the commit 786a44d to get more accurate results

@@            Coverage Diff             @@
##            maint    #5609      +/-   ##
==========================================
- Coverage   90.22%   84.50%   -5.72%     
==========================================
  Files         299      296       -3     
  Lines       42232    42212      -20     
==========================================
- Hits        38102    35672    -2430     
- Misses       4130     6540    +2410     

Impacted Files	Coverage Δ
datalad/distribution/tests/test_uninstall.py	85.58% <ø> (-14.42%)	⬇️
datalad/support/tests/test_annexrepo.py	95.47% <100.00%> (-2.02%)	⬇️
datalad/support/tests/utils.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/metadata/extractors/tests/test_audio.py	19.35% <0.00%> (-80.65%)	⬇️
datalad/metadata/extractors/xmp.py	12.96% <0.00%> (-79.63%)	⬇️
datalad/metadata/extractors/exif.py	18.75% <0.00%> (-78.13%)	⬇️
datalad/metadata/extractors/image.py	19.35% <0.00%> (-77.42%)	⬇️
datalad/metadata/extractors/audio.py	20.00% <0.00%> (-77.15%)	⬇️
datalad/metadata/extractors/tests/test_exif.py	24.00% <0.00%> (-76.00%)	⬇️
datalad/metadata/extractors/tests/test_xmp.py	26.92% <0.00%> (-73.08%)	⬇️
... and 97 more

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