Avoid implementation defined behavior in a test.

Summary:
num_put::put uses %p for pointer types, but the exact format of %p is
implementation defined behavior for the C library. Compare output to
snprintf for portability.

Reviewers: EricWF, mclow.lists

Reviewed By: EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D29197

llvm-svn: 293926