debugger: `restart` with custom debug port (bugfix)

Fixed a bug in debugger repl where `restart` command did not work
when a custom debug port was specified via command-line option
`--port={number}`.

To improve troubleshooting of similar problems in the future, debugger repl
now prints the port it is connecting to.

File `test/simple/helper-debugger-repl.js` was extracted from `test/simple/test-debugger-repl.js`