MXS-173 Test for throttling filter

The test may fail if the client/maxscale/mariadb combo is too slow.
TODO, maybe: today I saw mysql_query() hang (in a poll) when maxscale
disconnected. Is there something that can be done about that?
I added a source directory, base, for stuff that should become part of a common
utility library in the future.

maxscale-system-test/base/appexception.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2018 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#pragma once
+
+#include <stdexcept>
+#include <sstream>
+
+namespace base
+{
+
+// TODO Add back trace.
+class AppException : public std::runtime_error
+{
+public:
+    AppException(const std::string& msg, const std::string& file,
+                 int line) :
+        std::runtime_error(msg), m_file(file), m_line(line)
+    {}
+private:
+    std::string m_file;
+    int m_line;
+};
+} //base
+
+#define DEFINE_EXCEPTION(Type) \
+    struct Type : public base::AppException { \
+        Type(const std::string& msg, const char* file, \
+            int line) : \
+            AppException(msg, file, line) {}}
+
+#define THROW(Type, msg_str) do {\
+        std::ostringstream os; \
+        os << __FILE__ << ':' << __LINE__ << '\n' << msg_str; \
+        throw Type(os.str(), __FILE__, __LINE__);} while(false)
+
+