Hide utility class constructors

Update all utility classes so that they have a private constructor. This
prevents users from accidentally creating an instance, when they should
just use the static methods directly.

Issue gh-8945

web/src/test/java/org/springframework/security/test/web/reactive/server/WebTestClientBuilder.java

@@ -34,7 +34,10 @@ import org.springframework.web.server.WebFilter;
  * @since 5.0
  *
  */
-public class WebTestClientBuilder {
+public final class WebTestClientBuilder {
+
+	private WebTestClientBuilder() {
+	}
 
 	public static Builder bindToWebFilters(WebFilter... webFilters) {
 		return WebTestClient.bindToController(new Http200RestController()).webFilter(webFilters).configureClient();

web/src/test/java/org/springframework/security/web/authentication/preauth/x509/X509TestUtils.java

@@ -25,7 +25,10 @@ import java.security.cert.X509Certificate;
  *
  * @author Luke Taylor
  */
-public class X509TestUtils {
+public final class X509TestUtils {
+
+	private X509TestUtils() {
+	}
 
 	/**
 	 * Builds an X.509 certificate. In human-readable form it is: