Replace failing test_invalid_subconfig subtest

[achaikou]

test_invalid_subconfig began failing due to changes in the environment.

Test happened to verify that a validation error is thrown when all directories on the path are unwritable.

1. this is difficult to reproduce in controlled test setup.
2. intuitively one would expect that it is enough for provided directory to be writable, yet check is implemented to verify all directories along the path. Reason for this is unclear.
3. it is unclear if test on purpose used a path with all directories unwritable or if it was an accident.

As expected behavior is unknown, create a x-failing replacement test for behavior to be verified in the future.

• 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 'just rapid-tests')

When applicable

• When screenshots are changed: Review screenshot-PR in ert-testdata,
  merge screenshot-PR in ert-testdata before merging this PR.
• 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: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.83%. Comparing base (ae2f9f1) to head (6c5463b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14055      +/-   ##
==========================================
- Coverage   91.85%   91.83%   -0.02%     
==========================================
  Files         481      480       -1     
  Lines       33342    33337       -5     
==========================================
- Hits        30626    30615      -11     
- Misses       2716     2722       +6     

Flag	Coverage Δ
cli-tests	36.34% <ø> (+<0.01%)	⬆️
fuzz	44.11% <ø> (-0.02%)	⬇️
gui-tests	58.38% <ø> (-0.06%)	⬇️
performance-and-unit-tests	80.38% <ø> (-0.02%)	⬇️
test	46.46% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 2 files with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

---

_{Comparing achaikou:everest_robust_test (6c5463b) with main (91d37b4)}