fix schema test and move dropSchema to AbstractDialect

The schema.test.js added yesterday was using before/after in the
test cases when this.sequelize was not guaranteed to be present yet.
This also moves dropSchema to the AbstractDialect, currently the
code simply assumed you had this implemented in your dialect
Merge pull request #2665 from mbroadst/fix-schema-test-move-drop-schema

fix schema test and move dropSchema to AbstractDialect