stdint.h already has macros for this. The spec says that, in C++,
__STDC_CONSTANT_MACROS is needed, so define it for bytestring_test.cc.
Chromium seems to use these macros without trouble, so I'm assuming we
can rely on them.

Change-Id: I56d178689b44d22c6379911bbb93d3b01dd832a3
Reviewed-on: https://boringssl-review.googlesource.com/6510
Reviewed-by: Adam Langley <agl@google.com>