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.


mysql-test/mysql-test-run.sh:
  If mysql-test-run spawns the server by itself, USE_RUNNING_SERVER will be set to 0 instead of "".
      If the option "--extern" was detected, USE_RUNNING_SERVER will be set to 1 instead of "1".
      Several comparisons have to be changed
           [ -z "$USE_RUNNING_SERVER" ] --> [ $USE_RUNNING_SERVER -eq 0 ]
           [ -n "$USE_RUNNING_SERVER" ] --> [ $USE_RUNNING_SERVER -eq 1 ]
      USE_RUNNING_SERVER will be exported.