NF: test can pick up extension test suites and run them too

[mih]

Without requiring a user to keep track of what extensions are installed
on a particular system.

@bpoldrack This implements your idea. The neuroimaging extensions already registers its module as tests.

[codecov]

Codecov Report

Merging #2370 into master will decrease coverage by 0.01%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master    #2370      +/-   ##
==========================================
- Coverage   89.99%   89.97%   -0.02%     
==========================================
  Files         293      293              
  Lines       33423    33430       +7     
==========================================
  Hits        30079    30079              
- Misses       3344     3351       +7

Impacted Files	Coverage Δ
datalad/interface/test.py	65% <22.22%> (-27.31%)	⬇️
datalad/support/tests/utils.py	95.23% <0%> (-4.77%)	⬇️

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[mih]

I cannot see why the buildbots are that unhappy.