Supplying --skip-rpl to mysql-test-run.pl would always disable the
  slaves, but those slaves may still be needed for the federated tests.
  Now we only disable the slaves when they are not used by any of the
  tests.


mysql-test/mysql-test-run.pl:
  Decide whether to set up and start slave databases based entirely on
  which tests are used, and don't allow --skip-rpl to ruin the party,
  since other tests than replication rely on the slaves (namely federated).