Adding fix to make the test_runner functions pub

[TornaxO7]

I always got the following error message when I tried chapter 4:

error[E0463]: can't find crate for `test`

For more information about this error, try `rustc --explain E0463`.
error: could not compile `os` due to previous error
warning: build failed, waiting for other jobs to finish...
error: build failed

After comparing your source code on branch post_04 with my code, I found out that the issue was function signature of test_runner: My signature was without pub. Adding the pub fixed it for me.

HINT: I don't really know if this PR is "necessary" because I couldn't find anyone else in the comments who got the same problem. Could it be that it just happened on my machine?

Greetings from Kerlsruhe by the way :)

[phil-opp]

Sorry, this somehow slipped through my notifications. Thanks for submitting!