[JENKINS-41511] Add a test showing the problem

test/src/test/groovy/jenkins/bugs/Jenkins41511Test.java

+package jenkins.bugs;
+
+import org.junit.BeforeClass;
+import org.junit.Rule;
+import org.junit.Test;
+import org.jvnet.hudson.test.JenkinsRule;
+
+import hudson.security.HudsonPrivateSecurityRealm;
+import jenkins.model.Jenkins;
+
+public class Jenkins41511Test {
+
+    @BeforeClass
+    public static void setUpClass() {
+        System.setProperty(Jenkins.class.getName()+".slaveAgentPort", "10000");
+        System.setProperty(Jenkins.class.getName()+".slaveAgentPortEnforce", "true");
+    }
+
+    @Rule
+    public JenkinsRule j = new JenkinsRule();
+
+    @Test
+    public void configRoundTrip() throws Exception {
+        Jenkins.getInstance().setSecurityRealm(new HudsonPrivateSecurityRealm(true, false, null));
+        j.submit(j.createWebClient().goTo("configureSecurity").getFormByName("config"));
+    }
+}