debugger: `restart` with custom debug port

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