MXS-2563: Add test case

Added a test case that reproduces the debug assertion with the old code
and verifies that it is fixed with the new one.

maxscale-system-test/mxs2563_concurrent_slave_failure.cpp

@@ -0,0 +1,35 @@
+/**
+ * MXS-2563: Failing debug assertion at rwsplitsession.cc:1129 : m_expected_responses == 0
+ * https://jira.mariadb.org/browse/MXS-2563
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char* argv[])
+{
+    TestConnections test(argc, argv);
+
+    test.maxctrl("alter monitor MariaDB-Monitor monitor_interval 99999");
+
+    auto conn = test.maxscales->rwsplit();
+    conn.connect();
+    conn.query("SET @a = (SELECT SLEEP(1))");
+
+    std::thread thr(
+        [&test]() {
+            sleep(5);
+            test.repl->stop_node(2);
+            test.repl->stop_node(3);
+            sleep(5);
+            test.repl->start_node(2);
+            test.repl->start_node(3);
+        });
+    // Should go to server2
+    conn.query("SELECT SLEEP(20)");
+    thr.join();
+
+
+    test.maxctrl("alter monitor MariaDB-Monitor monitor_interval 1000");
+
+    return test.global_result;
+}