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/env.h

@@ -0,0 +1,46 @@
+/*
+ * 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 <string>
+
+namespace base
+{
+
+/* Environment variable. Usage:
+ * Env user{"USER"};
+ * std::string home = Env{"HOME"};
+ * An environment variable can be empty() but
+ * still is_defined().
+ */
+class Env : public std::string
+{
+public:
+    Env(const std::string& name) : m_is_defined(false)
+    {
+        if (const char* var = getenv(name.c_str()))
+        {
+            m_is_defined = true;
+            this->assign(var);
+        }
+    }
+
+    bool is_defined() const
+    {
+        return m_is_defined;
+    }
+private:
+    bool m_is_defined;
+};
+
+} // base