Periodic testing of envoy-security@googlegroups.com

We should periodically verify that a report filed to envoy-security@googlegroups.com makes its way past e-mail filters/redirects etc. to a human being in the Envoy security team. This is motivated by the wish to avoid something like https://github.com/nodejs/security-wg/issues/454#issuecomment-481919759 occurring and the blackholing of vulnerability reports.

Action item for CVE-2019-9900