Add code review checklist item for flaky tests

What does this MR do and why?

This MR adds an item to the code review acceptance checklist explaining that code introducing flaky tests should either fix the tests, or provide evidence of why the flaky test can be ignored.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.