Remove nose-based testing utils and possibility to test extensions using nose

[yarikoptic]

As promised in #7260 against maint.

[codeclimate]

Code Climate has analyzed commit 1cddc14 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Base: 88.72% // Head: 91.58% // Increases project coverage by +2.85% 🎉

Coverage data is based on head (1cddc14) compared to base (7afb741).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7261      +/-   ##
==========================================
+ Coverage   88.72%   91.58%   +2.85%     
==========================================
  Files         326      324       -2     
  Lines       44420    43033    -1387     
  Branches     5919     5771     -148     
==========================================
  Hits        39413    39413              
+ Misses       4991     3605    -1386     
+ Partials       16       15       -1     

Impacted Files	Coverage Δ
datalad/conftest.py	91.58% <0.00%> (-1.87%)	⬇️
datalad/utils.py	87.94% <0.00%> (ø)
datalad/support/tests/test_sshconnector.py	100.00% <0.00%> (+1.22%)	⬆️

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[yarikoptic]

thank you @adswa for approval, let's proceed and see if any remnants will be picked up by datalad-extensions testing ;)

[yarikoptic-gitmate]

PR released in 0.19.0