chore: stage ktest module

[justinsb]

This should enable us to add a module much more easily, because go mod tidy will work.

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[justinsb]

This is intended to support #383 , which ends up needing a new shared library, but it proves very difficult to add a module and use it in one commit, even with go workspaces. I think creating a stub module (as here) should (1) work around the problem and (2) be a reasonable compromise in terms of breaking up commits.