test: fix flaky repl-timeout-throw

Don't disconnect the child until all exceptions are thrown.

Fixes: https://github.com/nodejs/node/issues/18659

PR-URL: https://github.com/nodejs/node/pull/18692
Fixes: https://github.com/nodejs/node/issues/18659


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>