mysql-test/t/variables.test, because:
 - mysql-test/suite/rpl/t/rpl_variables.test does not replicate anything,
so should not be in the rpl suite.
 - mysql-test/t/variables.test is the place for testing variable-related
problems and features.
 - I will soon commit a patch containing a test case that tests
replication of variables. It would be good if I could call the test case
mysql-test/suite/rpl/t/rpl_variables.test. I'm making place for that now.