test: refactor test-tls-reuse-host-from-socket

Replace booleans with `common.mustCall()`, migrate from `var` to
`const`, and apply minor formatting changes.

PR-URL: https://github.com/nodejs/node/pull/6756


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>