Clean up testcases issues that have duplicate discussions

After https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/438 and gitlab-org/gitlab-qa!421 (merged) test results should be reported as brief comments in a discussion when the error is the same.

Unfortunately, before those changes a lot of duplicate errors were reported, so most of the testcase issues have a lot of duplicates taking up space and making it harder to quickly find relevant results.

We should clean up the existing duplicate discussions.

Given that we haven't been using issues this way for long, the easiest option would be to close all the existing issues and start fresh.

But if we just close the issues then new ones won't be linked to the old ones automatically, so it might be best to

• write a one-off script to create a new issue for each test, link the new issue to the old one, and close the old issue.