test: refactor test-http-expect-continue

Use common.mustCall() where appropriate. Remove some logic that is not
required when common.mustCall() is used (incrementor/decrementor to make
sure everything is called the same number of times).

PR-URL: https://github.com/nodejs/node/pull/19625


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>