Test cases using common object (database,user,table,..) names could probably
  destroy customer data, when connecting to an already running server.
  
  This Changeset contains an auxiliary routine  include/testdb_only.inc" which
  should be sourced by tests doing such dangerous things. 
  The test case will get a "[skipped]" when the environment variable 
  USE_RUNNING_SERVER is not 0 .
  The modified mysql-test-run.sh sets USE_RUNNING_SERVER to 0, when 
  the "mysql-test-run" starts its own server, aka there was no "--extern"
  option.