test: guarantee test runs in test-readline-keys

Use common.mustCall() to guarantee that test functions (created by a
factory function) are run.

PR-URL: https://github.com/nodejs/node/pull/11023


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>