fix: Mark GdchCredentialsTestUtil test util class as public (#1877)

Author: lqiu96

google-auth-library-java/oauth2_http/javatests/com/google/auth/oauth2/GdchCredentialsTestUtil.java

@@ -5,7 +5,11 @@
 import java.net.URI;
 import java.util.Map;
 
-class GdchCredentialsTestUtil {
+/**
+ * This class marked public as it is used in showcase tests in sdk-platform-java to test GDCH:
+ * https://github.com/googleapis/sdk-platform-java/blob/3db61c211b757796a3e212c384487d0f2827322b/java-showcase/gapic-showcase/src/test/java/com/google/showcase/v1beta1/it/ITGdch.java#L215-L223
+ */
+public class GdchCredentialsTestUtil {
   public static void registerGdchCredentialWithMockTransport(
       GdchCredentials credentials,
       MockTokenServerTransport transport,