backfill test: RP initiated logout

- Add test coverage on the existing behavior described in
  #2589 where UAA attempts to
  log the user out of the external OIDC provider after a successful UAA
  logout (this is called the RP initiated logout)

[#184752215]

uaa/src/test/java/org/cloudfoundry/identity/uaa/integration/feature/OIDCLoginIT.java

@@ -88,6 +88,7 @@
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.endsWith;
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.startsWith;
 import static org.junit.Assert.assertEquals;
@@ -181,6 +182,7 @@ public void setUp() throws Exception {
         config.setTokenKeyUrl(new URL(urlBase + "/token_key"));
         config.setIssuer(urlBase + "/oauth/token");
         config.setUserInfoUrl(new URL(urlBase + "/userinfo"));
+        config.setLogoutUrl(new URL(urlBase + "/logout.do"));
 
         config.setShowLinkText(true);
         config.setLinkText("My OIDC Provider");
@@ -558,6 +560,14 @@ public void testResponseTypeRequired() {
         assertThat(webDriver.getCurrentUrl(), containsString("error_description=Missing%20response_type%20in%20authorization%20request"));
     }
 
+    @Test
+    public void successfulUaaLogoutTriggersExternalOIDCProviderLogout() {
+        validateSuccessfulOIDCLogin(zoneUrl, testAccounts.getUserName(), testAccounts.getPassword());
+
+        webDriver.get(baseUrl);
+        Assert.assertThat("URL validation failed", webDriver.getCurrentUrl(), endsWith("/login"));
+    }
+
     private String getRefreshTokenResponse(ServerRunning serverRunning, String refreshToken) {
         MultiValueMap<String, String> formData = new LinkedMultiValueMap<>();
         formData.add("client_id", zoneClient.getClientId());