Assume that the test database exists

The tests should not drop the test database and then recreate it. This
adds an unnecessary burden on replication which will cause false positives
when replication is not fast enough.

This is commit `2410c23e6664bef7e449dfd5a767121f309559fe` from develop
cherry-picked to 2.1.

maxscale-system-test/bug649.cpp

@@ -126,8 +126,7 @@ int main(int argc, char *argv[])
 
     /** Clean up */
     Test->repl->connect();
-    execute_query(Test->repl->nodes[0], "DROP DATABASE test");
-    execute_query(Test->repl->nodes[0], "CREATE DATABASE test");
+    execute_query(Test->repl->nodes[0], "DROP TABLE IF EXISTS t1;");
 
     int rval = Test->global_result;
     delete Test;