test-https-agent-additional-options is invoked with a command-line flag.
However, there is an equivalent option that can be enabled within the
code. Do that instead.

In test-https-agent-additional-options, use arrow functions for
anonymous callbacks. Replace a use of `this` with the relevant constant.

Move callback to location where it is less confusing.

test-https-agent-additional-options can occasionally fail if a socket
closes before the checks near the end of the test. Modify the test to
check the freeSockets pool after each request.

Fixes: nodejs#24449