disable one BitsTest on 32-bit
disable one BitsTest on 32-bit

Summary:
One of the tests assumes `rand64` and `uintptr_t` are the same size,
which is only true on architectures with 64-bit word sizes.

Reviewed By: yfeldblum

Differential Revision: D34629203

fbshipit-source-id: ce91d1ac1d02b89335d611a6ad98d0770f35a29e

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly