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}.

File test/simple/helper-debugger-repl.js was extracted
from test/simple/test-debugger-repl.js
debugger: print port number when connecting to debuggee

To improve troubleshooting of debugger problems in the future,
the debugger repl now prints the port it is connecting to.