MXS-2520: Add test case

The test reproduces the problem and verifies that the fix works.

maxscale-system-test/mxs2520_master_read_reconnect.cpp

@@ -0,0 +1,34 @@
+/**
+ * MXS-2520: Allow master reconnection on reads
+ * https://jira.mariadb.org/browse/MXS-2520
+ */
+
+#include "testconnections.h"
+
+int main(int argc, char* argv[])
+{
+    TestConnections test(argc, argv);
+
+    test.maxscales->connect();
+    std::thread thr([&]() {
+                        sleep(5);
+                        test.tprintf("block node 0");
+                        test.repl->block_node(0);
+                        test.tprintf("wait for monitor");
+                        test.maxscales->wait_for_monitor(2);
+                        test.tprintf("unblock node 0");
+                        test.repl->unblock_node(0);
+                    });
+
+    test.set_timeout(60);
+    test.tprintf("SELECT SLEEP(10)");
+    test.try_query(test.maxscales->conn_rwsplit[0], "SELECT SLEEP(10)");
+    test.stop_timeout();
+
+    test.tprintf("disconnect");
+    test.maxscales->disconnect();
+    test.tprintf("join");
+    thr.join();
+
+    return test.global_result;
+}