test(system-defaults): cover custom-file, backups, and GC-hook registration

system-defaults.el had no coverage for its settings, only its functions (in test-system-defaults-functions.el) and the vc-follow-symlinks default. I added three settings smoke tests: custom-file is redirected to a temp trashbin rather than the repo, backups land under user-emacs-directory/backups, and the minibuffer GC hooks are actually wired onto the minibuffer hooks.

I pulled the sandbox loader the vc-follow-symlinks test had inline into tests/testutil-system-defaults.el so both files share one copy. The backups test clears cj/backup-directory before loading — it's a defvar, so once an earlier test loads the module it keeps that first sandbox path and won't recompute, which made the assertion fail until I forced the recompute.

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