Add ps-mode-tests.el and remove a compilation warning

* lisp/progmodes/ps-mode.el (ps-mode-octal-region): Remove a
string-make-unibyte that apparently had no effect here.
* test/lisp/progmodes/ps-mode-tests.el: New file.
1 job for master in 6 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #2138
test-all

00:00:06

 
Name Stage Failure
failed
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:e1de74e67cc70422153600785c4ce4c29843a0e59e18b634c52ca133c564437f for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:0s)