Use cl-letf instead of unwind-protect in a test

* test/lisp/cus-edit-tests.el (cus-edit-tests-customize-saved/show-obsolete):
Good use case for cl-letf, so use it.
Suggested by Stefan Monnier <monnier@iro.umontreal.ca> in:
https://lists.gnu.org/archive/html/emacs-devel/2020-11/msg00914.html
1 job for master in 120 minutes and 2 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #8916
test-all

02:00:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/eshell/eshell-tests.log
ELC lisp/faces-tests.elc
GEN lisp/faces-tests.log
ELC lisp/ffap-tests.elc
GEN lisp/ffap-tests.log
ELC lisp/filenotify-tests.elc
GEN lisp/filenotify-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 2h0m0s seconds