test: fix and improve debug-break-on-uncaught

This test runs based on a expectation that the stderr will get the
string 'Debugger listening on port'. But the actual message printed
to stderr has changed to 'Debugger listening on host:port'. So the
the actuals tests did not even start and eventually timeout.

Apart from that, changed `var`s to `let`s or `const`s.

Refs: https://github.com/nodejs/node/issues/10361
PR-URL: https://github.com/nodejs/node/pull/10370


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