test: fix and improve debugger-client test

This test expects the string 'Debugger listening on port' on stderr and
since the message has been changed to 'Debugger listening on host:port'
this was failing always.

Apart from that, this test expects the main script's name to be
`src/node.js`, but that has been renamed to `lib/internal/node.js` and
then to `lib/internal/bootstrap_node.js`. So, the script name has been
updated.

Apart from that, using `const` instead of `var` wherever possible.

Refer: https://github.com/nodejs/node/issues/10361

PR-URL: https://github.com/nodejs/node/pull/10371



Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>