MXS-2207 Add test that reveals problem

With qc_mysqlembedded statements like

    SET STATEMENT ... FOR stmt;

are always classified as READ, although their type should be
that of stmt.

query_classifier/test/maxscale.test

@@ -112,3 +112,6 @@ SET STATEMENT max_statement_time=30 FOR SELECT seq FROM seq_0_to_100000;
 
 # MXS-1935
 PREPARE a FROM @sql;
+
+# MXS-2207
+SET STATEMENT max_statement_time=30 FOR UPDATE tbl SET a = 42;