Fix samples test suite execution and failing tests

Closes gh-1324

samples/demo-authorizationserver/samples-demo-authorizationserver.gradle

@@ -30,3 +30,7 @@ dependencies {
 	testImplementation "org.junit.jupiter:junit-jupiter"
 	testImplementation "net.sourceforge.htmlunit:htmlunit"
 }
+
+test {
+	useJUnitPlatform()
+}

samples/demo-authorizationserver/src/test/java/sample/DemoAuthorizationServerApplicationTests.java

@@ -75,7 +75,8 @@ public class DemoAuthorizationServerApplicationTests {
 
 		this.webClient.getOptions().setThrowExceptionOnFailingStatusCode(false);
 		WebResponse signInResponse = signIn(page, "user1", "password").getWebResponse();
-		assertThat(signInResponse.getStatusCode()).isEqualTo(HttpStatus.NOT_FOUND.value());	// there is no "default" index page
+
+		assertThat(signInResponse.getStatusCode()).isEqualTo(HttpStatus.BAD_REQUEST.value());	// there is no "default" index page
 	}
 
 	@Test
@@ -86,7 +87,7 @@ public class DemoAuthorizationServerApplicationTests {
 
 		HtmlElement alert = loginErrorPage.querySelector("div[role=\"alert\"]");
 		assertThat(alert).isNotNull();
-		assertThat(alert.getTextContent()).isEqualTo("Bad credentials");
+		assertThat(alert.asNormalizedText()).isEqualTo("Invalid username or password.");
 	}
 
 	@Test