test: improve test-debugger-util-regression

Avoid the `exit` command to be sent more than once. It prevents from
undesired errors emitted on `proc.stdin`.
Remove the watchdog timer so the test does not fail in case it takes
longer to complete.

PR-URL: https://github.com/nodejs/node/pull/9490


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