ci(mf6.yml): include more mf6 autotests

[wpbonelli]

• previously only files named test_gw*.py were run
• run the full mf6 test set minus (as before) example model / regression tests (test_z*.py)
• may catch more mf6 integration issues without much extra time (previously mf6.yml took ~15min, now ~20)
• prepares also for differently-named models e.g. prt

[codecov]

Codecov Report

Merging #1983 (be92bec) into develop (f3b9410) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop   #1983   +/-   ##
=======================================
  Coverage     72.7%   72.7%           
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  Files          258     258           
  Lines        58829   58829           
=======================================
  Hits         42788   42788           
  Misses       16041   16041