fix tests for removed implicit macOS dependency

- generate-cask-ci-matrix: fallback to MACOS_RUNNERS.dup instead of
  RUNNERS.dup when no macOS version filter matches, preventing Linux
  runners from leaking into macOS-only cask CI
- info_spec: remove expectations of implicit macOS >= 10.15 requirement
  from casks without explicit depends_on :macos
- installer_spec: use cask with explicit depends_on macos for
  requirement stub test