The default role permissions weren't being saved
to a separate variable which would have not restored
them properly. This was a problem in various other
tests, potentially causing side-effects in other
flaky tests as well.
We fix that, as well as clean up the for loop
a bit and mention how many messages were recieved
in the failure message so that it becomes clear
whether no messages were recieved, or atleast some
were received.
https://mattermost.atlassian.net/browse/MM-62079
```release-note
NONE
```
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