test(test): add iperf migration continuity checks to release e2e

[hayer969]

The release test confirmed that iperf kept running across the upgrade, the added checks make the test actually prove the TCP session was not interrupted by the migration.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

section: test
type: feature
summary: "The release upgrade e2e test now verifies that a long-running TCP session survives virtual machine live migration during the module upgrade."
impact_level: low