test: add basic cctest for base64.h

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.

PR-URL: https://github.com/nodejs/node/pull/12238
Refs: https://github.com/nodejs/node/pull/12146#issuecomment-291559685


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>