Move proper-list-p tests to fns-tests.el

This follows the move of proper-list-p from lisp/subr.el to
src/fns.c in 2018-07-24T15:58:46-07:00!eggert@cs.ucla.edu.
* test/lisp/subr-tests.el (subr-tests--proper-list-p): Move from
here...
* test/src/fns-tests.el (test-proper-list-p): ...to here.
1 job for master in 50 minutes and 11 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #1320
test-all

00:50:11

 
Name Stage Failure
failed
test-all Test
Makefile:313: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:288: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:962: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1