tox: support posargs, via TEST_ARGS in Makefile

[blueyed]

No description provided.

[codecov]

Codecov Report

Merging #983 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #983   +/-   ##
=======================================
  Coverage   83.46%   83.46%           
=======================================
  Files          28       28           
  Lines        1578     1578           
  Branches      235      235           
=======================================
  Hits         1317     1317           
  Misses        192      192           
  Partials       69       69

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[matthiask]

Thanks!