I've configured rocketchat notifier with the option "Trust Server Certificate" set, and the test connection works, giving the message
"you're all set on [domain]" in my rocket chat channel.
I've configured the rocketchat plugin for the build in question and checked to be notified on all event types, however no message is received and in the logs there is the message:
"Could not send rocket message"

I updated the relevant log message to include the actual error, and reloaded the plugin which now gives:

Build completed. Checking for rocket notifiers
May 04, 2018 9:00:14 AM INFO jenkins.plugins.rocketchatnotifier.ActiveNotifier completed
Could not send rocket message: Please check if the server API domain:2015/api/v1/login is correct: javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

It appears that the test is not checking for a certificate, but to actually send a message it still is.