C99ify the test programs.

[icculus]

I'm going to add a low-priority task to the 3.x milestone to go through the test programs and move variable declarations around and convert /* comments */ to // comments where it makes sense, but it's not urgent at all. Just a nice chance for little cleanups at some point.

Originally posted by @icculus in #14588

...and here is that task. :) I'm assigning myself, but anyone can feel free to steal this if they get to it first!