Improved zero downtime test script - increased waiting time, avoided race condition, better logging

[mluk-sap]

Description

Changes proposed in this pull request:

• Improved zero downtime test script
• increased timeout when waiting for the URL to be available (it was the major factor of test flakiness)
• added 2 seconds delay to reduce the probability of race condition when test finishes
• added additional check whether the number of requests is too low (<10)
• more verbose logging, logging failed responses, logging the thread number, etc.

Pre-Merge Checklist

• As a PR reviewer, verify code coverage and evaluate if it is acceptable.

Related issues