A test pins app-note moderation: a banned visitor's notes leave the app's list, and unbanning brings them back #6792

src/core/app_notes.ts holds the notes visitors leave in an app, with hideNote, banVisitor, unbanVisitor and bannedNotes; none is called by a test directly.

Done when a new test file proves, with two visitors' notes on one app and one on another app:

• [ ] notesOfApp lists all of the app's notes and none of the other app's.
• [ ] hideNote removes one note from notesOfApp and from the visitor's own notesOf as the module documents.
• [ ] banVisitor removes that visitor's notes from notesOfApp and lists them in bannedNotes, only for that app; unbanVisitor restores them.
• [ ] the limit arguments cap each list.

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