TST: mark test_source_candidate_subdataset as @slow

[yarikoptic]

See #6412 for more background.
The actual reason was not discovered, but the test became slower in late Jan,
crossing the line of 30seconds. We are marking it as slow to mitigate
false positive CI fails on PRs.

[codeclimate]

Code Climate has analyzed commit c79758a and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #6429 (c79758a) into master (cc84266) will increase coverage by 0.43%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6429      +/-   ##
==========================================
+ Coverage   89.57%   90.01%   +0.43%     
==========================================
  Files         345      345              
  Lines       43032    43313     +281     
==========================================
+ Hits        38545    38987     +442     
+ Misses       4487     4326     -161     

Impacted Files	Coverage Δ
datalad/distribution/tests/test_get.py	100.00% <100.00%> (ø)
datalad/_version.py	46.99% <0.00%> (-53.01%)	⬇️
datalad/customremotes/tests/test_archives.py	89.28% <0.00%> (-0.65%)	⬇️
datalad/support/tests/test_annexrepo.py	97.97% <0.00%> (+0.13%)	⬆️
datalad/support/gitrepo.py	90.37% <0.00%> (+0.16%)	⬆️
datalad/distribution/tests/test_install.py	100.00% <0.00%> (+0.20%)	⬆️
datalad/distribution/tests/test_dataset.py	99.70% <0.00%> (+0.29%)	⬆️
datalad/core/distributed/tests/test_clone.py	97.57% <0.00%> (+0.36%)	⬆️
datalad/local/add_archive_content.py	88.03% <0.00%> (+0.42%)	⬆️
datalad/support/network.py	87.22% <0.00%> (+0.44%)	⬆️
... and 40 more

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[mih]

Thx!