test: fix flaky test-https-timeout

Remove `setTimeout()` in test and instead rely on `common.mustCall()` on
a `timeout` event handler.

The test was flaky on CI. The flakiness was replicable by running the
test under load. This version, in contrast, is robust under load.

Took the opportunity to do some `var` -> `const` while refactoring.

PR-URL: https://github.com/nodejs/node/pull/10404


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: James M Snell <jasnell@gmail.com>