build: use --serial in 'just test_file'

[cklin]

Some tests require the --serial flag to pass.

Risk assessment

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• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull Request Overview

This PR modifies the test execution command to add the --serial flag for running individual test files, ensuring that tests that require sequential execution can pass properly.

[mbg]

Suggestion: add a command for running an individual test to package.json and then call that from here?