Refactor everest tmpdir fixture

[frode-aarstad]

Issue
Resolves #8860

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.53%. Comparing base (b505d33) to head (b6a7f62).
Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8885      +/-   ##
==========================================
+ Coverage   91.37%   91.53%   +0.16%     
==========================================
  Files         344      345       +1     
  Lines       21119    21252     +133     
==========================================
+ Hits        19297    19453     +156     
+ Misses       1822     1799      -23     

Flag	Coverage Δ
cli-tests	39.59% <ø> (+0.06%)	⬆️
gui-tests	73.37% <ø> (-0.11%)	⬇️
performance-tests	50.17% <ø> (+0.02%)	⬆️
unit-tests	80.30% <ø> (+0.20%)	⬆️

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[verveerpj]

LGTM. Small comments.