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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test https

Description of change