MXS-1503: Test that no extra slaves are taken into use

Added a test that verifies that the slave connection count restrictions
work.

maxscale-system-test/mxs1503_extra_slaves.cpp

@@ -0,0 +1,45 @@
+/**
+ * MXS-1503: Make sure no extra slaves are taken into use
+ *
+ * https://jira.mariadb.org/browse/MXS-1503
+ */
+#include "testconnections.h"
+#include <vector>
+#include <thread>
+#include <iostream>
+
+void query(MYSQL* conn, std::string q)
+{
+    execute_query(conn, "%s", q.c_str());
+    mysql_close(conn);
+}
+
+int main(int argc, char** argv)
+{
+    TestConnections test(argc, argv);
+    std::vector<std::thread> connections;
+
+    test.maxscales->connect();
+
+    auto original_row = get_row(test.maxscales->conn_rwsplit[0], "SELECT @@server_id");
+
+    for (int i = 0; i < 10; i++)
+    {
+        connections.emplace_back(query, test.maxscales->open_rwsplit_connection(), "SELECT SLEEP(10)");
+        sleep(1);
+        auto row = get_row(test.maxscales->conn_rwsplit[0], "SELECT @@server_id");
+        test.assert(row == original_row, "Value of @@server_id should not change: %s", row.at(0).c_str());
+    }
+
+    for (auto& a: connections)
+    {
+        a.join();
+        auto row = get_row(test.maxscales->conn_rwsplit[0], "SELECT @@server_id");
+        test.assert(row == original_row, "Value of @@server_id should not change: %s", row.at(0).c_str());
+    }
+
+
+    test.maxscales->disconnect();
+
+    return test.global_result;
+}