ci: an edit to examples/backups/_lib selects no e2e suite #4626

An edit to a shared file under examples/backups/_lib/ selects no e2e suite, so a PR that changes only that file passes the E2E gate with nothing run.

hack/select-e2e.sh maps examples/backups/<app>/* to suites by the directory name. The shared wait helpers for every backup walkthrough are moving into examples/backups/_lib/wait-helpers.sh, and _lib maps to an app with no suite, so the selection comes out empty. The same applies to hack/select-install.sh if it keys on the same path.

Fix shape: a rule that maps examples/backups/_lib/* to every suite that runs a backup walkthrough, or to the full suite if that set can't be derived reliably, with a select-e2e test for it.