What would you like to be added/modified:

To improve project stability, we need more tests to cover corner cases.
And the code coverage is around 50% currently, we need to add more tests to improve it.

To improve case coverage, we may need a list of cases to track the work.
For code coverage, simply check bef

Environment

• VerneMQ Version: using docker image vernemq/vernemq:latest
• OS: Debian 9
• Erlang/OTP version (if building from source):
• Cluster size/standalone: standalone
• VerneMQ configuration (vernemq.conf) or the changes from the default

accept_eula=yes
allow_anonymous=off
log.console=console
log.console.level=debug
plugins.vmq_acl=off
plugins.vmq_diversity=on
plugins.v