Make fixtures cleanup after themselves

[verveerpj]

Issue
Resolves #3279

Approach
Make sure that fixtures that create the poly.py file clean it up after yielding.

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[berland]

Should we consider adding a test for this similar to https://github.com/equinor/subscript/blob/f41b90e0945ce1f5e838039e2f865a56dd3d6bf9/.github/workflows/subscript.yml#L74-L76 ?

[verveerpj]

Should we consider adding a test for this similar to https://github.com/equinor/subscript/blob/f41b90e0945ce1f5e838039e2f865a56dd3d6bf9/.github/workflows/subscript.yml#L74-L76 ?

Excellent idea, will do.

[codecov-commenter]

Codecov Report

Merging #3287 (f1b8e73) into main (b708b59) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3287   +/-   ##
=======================================
  Coverage   64.91%   64.91%           
=======================================
  Files         618      618           
  Lines       48504    48504           
  Branches     4368     4368           
=======================================
  Hits        31486    31486           
+ Misses      15565    15563    -2     
- Partials     1453     1455    +2     

Impacted Files	Coverage Δ
libres/lib/res_util/block_fs.cpp	52.08% <0.00%> (-0.47%)	⬇️
ert_gui/simulation/run_dialog.py	81.53% <0.00%> (+0.76%)	⬆️

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