RF: Replace Runner->WitlessRunner in some tests and helpers

[mih]

Does not aim to be complete.

[codecov]

Codecov Report

Merging #4855 into master will increase coverage by 0.01%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master    #4855      +/-   ##
==========================================
+ Coverage   89.64%   89.65%   +0.01%     
==========================================
  Files         289      289              
  Lines       40481    40523      +42     
==========================================
+ Hits        36289    36332      +43     
+ Misses       4192     4191       -1     

Impacted Files	Coverage Δ
datalad/core/distributed/tests/test_clone.py	94.09% <50.00%> (ø)
datalad/cmdline/helpers.py	67.44% <100.00%> (ø)
datalad/cmdline/tests/test_main.py	97.01% <100.00%> (+1.36%)	⬆️
datalad/core/local/tests/test_create.py	100.00% <100.00%> (ø)
datalad/distribution/tests/test_create_sibling.py	99.27% <100.00%> (ø)
datalad/distribution/tests/test_install.py	99.78% <100.00%> (-0.01%)	⬇️
datalad/tests/test_config.py	100.00% <0.00%> (ø)
datalad/support/gitrepo.py	89.40% <0.00%> (+0.10%)	⬆️
datalad/config.py	96.73% <0.00%> (+0.13%)	⬆️

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