Fix file-tests.el on MS-Windows

* test/lisp/files-tests.el
(files-tests-file-name-non-special-start-file-process): On
MS-Windows, wait till the process dies before returning from
the files-tests--with-temp-non-special macro, to ensure the
temporary directory is successfully deleted.
1 job for master in 90 minutes and 1 second (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #4095
test-all

01:30:01

 
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 1h30m0s seconds