debug client: connect after child is ready

We now wait to connect to the debuggee until we know that
its error stream has data, to ensure that the output message
"connecting..... ok" appears after "Debugger listening on port xyz"

This change fixes the following unit tests on Windows:
- test-debugger-repl.js
- test-debugger-repl-term.js
- test-debugger-repl-utf8.js