Make files-tests.el more robust

* test/lisp/files-tests.el (files-tests--with-temp-non-special)
(files-tests--with-temp-non-special-and-file-name-handler): Make
it more robust wrt parallel test jobs.  (Bug#55706)
1 job for master in 240 minutes and 4 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Normal
failed #46612
test-lisp-inotify

04:00:04

 
Name Stage Failure
failed
test-lisp-inotify Normal The script exceeded the maximum execution time set for the job
  GEN      lisp/ffap-tests.log
ELC lisp/tabify-tests.elc
GEN lisp/tabify-tests.log
ELC lisp/minibuffer-tests.elc
GEN lisp/minibuffer-tests.log
ELC lisp/format-spec-tests.elc
GEN lisp/format-spec-tests.log
ELC lisp/files-tests.elc
GEN lisp/files-tests.log
ERROR: Job failed: execution took longer than 4h0m0s seconds