CommunityMessages - Define more consistent behavior in the face of slow execution

CommunityMessages - Define more consistent behavior in the face of slow execution

This passes under some brutally slow scenarios:

```
for TIME_FUNC in natural frozen linear:500 linear:1250 prng:500 prng:666 prng:1000 prng:1500 ; do
  export TIME_FUNC;
  echo;
  echo "TIME_FUNC=$TIME_FUNC" ;
  env CIVICRM_UF=UnitTests phpunit6 tests/phpunit/CRM/Core/CommunityMessagesTest.php ;
done
```