network_traffic, zeek: run system tests sequentially

[mrodm]

Proposed commit message

Set system.parallel to false in the _dev/test/config.yml files of the network_traffic and zeek packages so their system tests run sequentially instead of in parallel.

Author's Checklist

• Confirm system tests for network_traffic and zeek pass reliably when run sequentially in CI

How to test this PR locally

Run the system tests for each package and confirm they complete without the flakiness/failures seen when run in parallel:

elastic-package test system -v --report-format xUnit --report-output file -C packages/network_traffic
elastic-package test system -v --report-format xUnit --report-output file -C packages/zeek

Related issues

[elastic-vault-github-plugin-prod]

🚀 Benchmarks report

To see the full report comment with /test benchmark fullreport

[infra-vault-gh-plugin-prod]

💚 Build Succeeded

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• Commit: fe20c6a

cc @mrodm