Filter `gameoverlayrenderer.so` from LD_PRELOAD

gameoverlayrenderer.so leads to a spammy and slow output while running steam-runtime-system-info.

Because of this we just remove it from the LD_PRELOAD before calling the all the helper subrocesses.

I have made the filter unconditionally because I'm not aware of any cases where we'd like to keep the overlay lib preloaded. If you prefer to have a boolean to control it, let me know.

I have not declared it with G_GNUC_INTERNAL because I wanted to add some tests for this new function and we were not able to test it in the tests that we already had. So I created a new simple tests/utils.c to do the job (calling the function directly).