A test pins who sees a sealed issue: `mayView`, `canSeeSealed` and `mayViewId`

src/core/sealed.ts hides sealed issues and everything about them; no test calls its predicates directly.

Done when a new test file, on a seeded repository, proves:

[ ] an unsealed issue is visible to anyone who can read the repository.
[ ] a sealed issue is visible to its author while active and to the people the module names; not to a stranger, a reader, a suspended author, or no user.
[ ] mayViewId gives the same answer for a run or event about that issue, and true for a null issue id.
[ ] sealedPreview (or the preview path) leaks no title or body to someone who may not view.

Test only; a bug it finds is fixed in the same pull.

A test pins who sees a sealed issue: `mayView`, `canSeeSealed` and `mayViewId` #6788