[racket-dev] check syntax now more likely to work on std libs

I've just pushed a change that makes check syntax more likely to run
on files in the plt tree. Specifically, the namespace that check
syntax creates no longer shares the GUI libraries which means it pulls
in a far smaller set of libraries overall. (Of course, run still pulls
those in, so running those files will continue to not work.)

This kind of change can lead to surprising bugs so if you see check
syntax failing in strange ways, please let me know.

Thanks,
Robby