Stop using deprecated Repo.add_submodule()

[mih]

This involves a slight change of the test, which no longer
uses subdataset addressing by submodule name (not path).
A situation that is not achievable via the normal API usage.

Ping datalad/datalad#6010

[codecov]

Codecov Report

Merging #161 (d8471fb) into master (c4c5bb2) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   86.37%   86.29%   -0.08%     
==========================================
  Files          17       17              
  Lines         932      927       -5     
==========================================
- Hits          805      800       -5     
  Misses        127      127              

Impacted Files	Coverage Δ
datalad_container/tests/test_run.py	77.18% <100.00%> (-0.75%)	⬇️

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