test: favor strictEqual() in addon test

Replace `assert.equal()` with `assert.strictEqual()` throughout
`addon/make-callback-recurse/test.js`.

PR-URL: https://github.com/nodejs/node/pull/6704


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>