This commit adds C++ tests for `base64_encode()` and `base64_decode()`
functions defined in `base64.h`.  The functionality is already being
tested indirectly in JavaScript tests for Buffer, but it won't hurt to
test the low-level functions too, especially given that they aren't only
used in the internal Buffer implementation, Chrome inspector protocol
support relies upon them too.

Refs: nodejs#12146 (comment)

Add accidentally omitted consts for pointers to allocated buffers.
This doesn't actually affect anything in this code, but make const,
not var ;)