Increase sleep in mxs812_2

The test appears to fail due to connection errors when it attempts to
check whether MaxScale is still alive. To offset the chance of the backend
server still refusing connections after the initial spike, the sleep
before the check was increased.

maxscale-system-test/mxs812_2.cpp

@@ -90,7 +90,7 @@ int main(int argc, char *argv[])
     }
 
     Test->stop_timeout();
-    sleep(5);
+    sleep(20);
     Test->check_maxscale_alive(0);
     Test->check_current_operations(0, 0);