Await everest start_server coroutine in test_everest_output

[DanSava]

• 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

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.95%. Comparing base (c9ae9b2) to head (744d302).

Files with missing lines	Patch %	Lines
src/everest/config_file_loader.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9442      +/-   ##
==========================================
+ Coverage   91.94%   91.95%   +0.01%     
==========================================
  Files         433      433              
  Lines       26748    26748              
==========================================
+ Hits        24593    24597       +4     
+ Misses       2155     2151       -4     

Flag	Coverage Δ
cli-tests	39.41% <ø> (ø)
everest-models-test	34.70% <0.00%> (ø)
gui-tests	72.18% <ø> (+<0.01%)	⬆️
integration-test	37.25% <0.00%> (+0.29%)	⬆️
performance-tests	51.98% <ø> (+0.01%)	⬆️
test	40.62% <50.00%> (-0.03%)	⬇️
unit-tests	74.14% <0.00%> (ø)

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[oyvindeide]

LGTM!