couch_auth_cache_tests failed is_opened assert

[wohali]

Failed on the 2.1.0-RC1 build on a single instance, so a flaky test?

• https://travis-ci.org/apache/couchdb/jobs/259187964
  • https://couchdb-vm2.apache.org/ci_errorlogs/travis-couchdb-259187964-2017-07-30T22:24:12.028847/couchlog.tar.gz

      couch_auth_cache_tests:233: should_close_old_db_on_auth_db_change...*failed*
in function couch_auth_cache_tests:'-should_close_old_db_on_auth_db_change/1-fun-0-'/1 (test/couch_auth_cache_tests.erl, line 234)
in call from couch_auth_cache_tests:'-should_close_old_db_on_auth_db_change/1-fun-3-'/1 (test/couch_auth_cache_tests.erl, line 234)
**error:{assert,[{module,couch_auth_cache_tests},
         {line,234},
         {expression,"is_opened ( DbName )"},
         {expected,true},
         {value,false}]}
  output:<<"">>