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

[totten]

Overview

The CommunityMessagesTest is producing failures with TIME_FUNC option of linear:500 (the value currently used in PR test runs) -- which is to say, it fails when the system runs very slowly (with some 500ms stalling during execution).

This gets it working 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

[civibot]

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[seamuslee001]

looks good to me and the specific tests pass