test,repl: add coverage for repl .clear+useGlobal

Add a test to cover situation where REPL is initialized with `useGlobal`
set to `true` and `.clear` is called. This adds coverage for code in
repl.js that is not currently covered.

Includes minor refactor of rocket functions in repl.js for concision.

PR-URL: https://github.com/nodejs/node/pull/10777


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