In Permalinks, "Latest Test Result" field is not even shown for builds not completed in "lastCompletedBuild/testReport/"

This is because "lastCompletedBuild" can indicate passed or failed build. When build is failed, obviously, test report does not get generated.

Ideally, "Latest Test Result" field in Permalinks must instead point to "lastSuccessfulBuild/testReport/" 

This is about the hyperlink under "Latest Test Result (no failures)" in your screenshot.

When build is successfully completed, it shows this field along with test results.
When build is failed completed, it does NOT show this field.
This brings inconsistency in the view. It would be good to continue to see the last test report.

Solution is to show hyperlink for "Latest Test Report" from "lastSuccessfulBuild/testReport/" , instead of "lastCompletedBuild/testReport/" .

Few screenshots to show this issue. These are not from same job.

When build is successfully completed, "Latest Test Report" is shown

When build is failed, +"Latest Test Report" is NOT shown+