For Parsoid, we have comprehensive tests. When you fix a bug, you are expected to add a test showing the bug, and verifying that you've fixed it. I don't think it would help us at all to have to write "Test Plan: run the test suite." for every commit.
There are a few issues which are not easily testable with our test infrastructure (which is motivation for us to improve it). Writing a "Test Plan" for those would be unobjectionable, and perhaps helpful. But they are the exception, not the rule.
Which is all to say that mandatory fields are annoying, and we should have the ability on a project-by-project basis to enable/disable these things. --scott