Revert "core/internal/features/ocr2: parallelize subtests..."

[erikburt]

Reverts changes from #14754 / ec35d77.

Motivation

We've been seeing a lot of failures from http://github.com/smartcontractkit/chainlink/v2/core/internal/features/ocr2/plugins. This change may have caused some instability with the go-conditional-tests.

More information: https://smartcontract-it.atlassian.net/browse/RE-3313?focusedCommentId=338207

Testing

I tested partial reverts of these changes: #15795

The full revert was the most consistent - https://github.com/smartcontractkit/chainlink/actions/runs/12435970239/job/34727487863?pr=15792

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RE-3313

[github-actions]

I see you updated files related to core. Please run pnpm changeset in the root directory to add a changeset as well as in the text include at least one of the following tags:

• #added For any new functionality added.
• #breaking_change For any functionality that requires manual action for the node to boot.
• #bugfix For bug fixes.
• #changed For any change to the existing functionality.
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• #internal For changesets that need to be excluded from the final changelog.
• #nops For any feature that is NOP facing and needs to be in the official Release Notes for the release.
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🎖️ No JIRA issue number found in: PR title, commit message, or branch name. Please include the issue ID in one of these.

[cl-sonarqube-production]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube