Speed up tests by reusing plugin manager

[eivindjahren]

Saves ~35s of collecting plugins on my machine.

• 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

[codspeed-hq]

CodSpeed Performance Report

Merging #9740 will improve performances by 21.61%

_{Comparing eivindjahren:avoid_recreating_plugin_manager (6ad06b0) with main (024fcba)}

Summary

⚡ 1 improvements
✅ 23 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	eivindjahren:avoid_recreating_plugin_manager	Change
⚡	test_direct_dark_performance_with_storage[gen_x: 20, sum_x: 20 reals: 10-summary-get_record_observations]	1.5 ms	1.2 ms	+21.61%

[berland]

🕐