tests: Don't include "config.h" in installed files
These tests get installed, so config.h may not be available. Do the same thing that gimarshallingtests.c and regress.c already do, and include config.h conditionally.
These tests get installed, so config.h may not be available. Do the same thing that gimarshallingtests.c and regress.c already do, and include config.h conditionally.