test: fix flaky test-http-client-timeout-with-data

test-http-client-timeout-with-data has failed here and there in CI on
FreeBSD and OS X. The test has a socket timeout set to 50ms and a timer
set for 100ms. However, they are not necessarily set in the same tick of
the event loop and their ordering is therefore not guaranteed.

Instead of using a timer, this change listens for an event on the
listener to know when the socket timeout has occurred and then runs the
code originally in the timer.

Additional refactoring: Replaced `process.on('exit', ...)` checks with
`common.mustCall()` and replaced usage of `assert.equal()` with
`assert.strictEqual()`.

PR-URL: https://github.com/nodejs/node/pull/10431


Reviewed-By: James M Snell <jasnell@gmail.com>