Save output of mysqlmon tests

Before the tests truncate the log, it can be copied to a backup file. This
allows the whole log to be inspected after the test is complete.

maxscale-system-test/fail_switch_rejoin_common.cpp

@@ -22,7 +22,8 @@ void get_output(TestConnections& test)
     {
         test.tprintf("MaxScale output:");
     }
-    output = test.maxscales->ssh_node_output(0, "cat /var/log/maxscale/maxscale.log && "
+    output = test.maxscales->ssh_node_output(0,
+             "cat /var/log/maxscale/maxscale.log | tee -a /var/log/maxscale/maxscale_backup.log && "
              "sudo truncate -s 0 /var/log/maxscale/maxscale.log",
              true, &ec);
     if (test.verbose)