test: cleanup test-tls-connect-given-socket.js

Changed vars to consts and lets, assert.equals to
assert.strictEquals and added common.mustCall around callbacks.
Switched to arrow functions.

PR-URL: https://github.com/nodejs/node/pull/8616


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>