Use TESTDIR env var

[bynect]

Should resolve the issues pointed by @zappolowski in #1373

[bynect]

@apprehensions is this solution ok for you?

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.37%. Comparing base (d2faa87) to head (9655f2c).
⚠️ Report is 176 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1376      +/-   ##
==========================================
+ Coverage   65.34%   65.37%   +0.02%     
==========================================
  Files          50       50              
  Lines        8326     8321       -5     
  Branches     1001      999       -2     
==========================================
- Hits         5441     5440       -1     
+ Misses       2885     2881       -4     

Flag	Coverage Δ
unittests	65.37% <100.00%> (+0.02%)	⬆️

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