Fix SSL test failure on Ubuntu

SERVER-44620 introduced an isDebian10 function to ssl_helpers.js that verifies Debian version from the file /etc/debian_version. This was under the assumption that this file only exists on Debian systems, and not derivatives. On Ubuntu 18.04, the file exists and contains "buster/sid", which causes ssl_alert_reporting.js to attempt the test with TLS 1.3, which is not supported on Ubuntu 18.04. This causes a test failure.