test: change var declarations, add mustCall check

In this test, I changed the var declarations to be either a let or a
const. For some of the callbacks, I added a mustCall check to ensure
that the functions have run. I also changed assert.equal() to
assert.strictEqual().

PR-URL: https://github.com/nodejs/node/pull/9962


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>