clear out the test db for each test, making tests more isolated

migrate/tests/fixture/database.py

@@ -117,6 +117,9 @@ class DB(Base):
 
     def _setup(self, url):
         self._connect(url)
+        # make sure there are no tables lying around
+        meta = MetaData(self.engine, reflect=True)
+        meta.drop_all()
 
     def _teardown(self):
         self._disconnect()