test: remove dependency on node-weak

Replace node-weak with a small hand-rolled add-on.  We can now drop
node-weak and nan, reducing the size of the source tree by about 750 kB
and the size of the tarball by about 150-300 kB.

PR-URL: https://github.com/nodejs/node/pull/11239


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