Temporarily disabled ExtensionRegistryFactoryTest

This test has started failing, and I believe the cause is a change in
class loading behavior in Java 11. As far as I know, that breaks the
test logic but there is nothing wrong with the non-test code. This
change temporarily disables the test so that we can unblock the 4.0
release while I work on a more permanent fix.

java/core/src/test/java/com/google/protobuf/ExtensionRegistryFactoryTest.java

@@ -30,6 +30,7 @@
 
 package com.google.protobuf;
 
+import org.junit.Ignore;
 import protobuf_unittest.NonNestedExtension;
 import protobuf_unittest.NonNestedExtensionLite;
 import java.lang.reflect.Method;
@@ -52,7 +53,12 @@ import junit.framework.TestSuite;
  *
  * <p>The test mechanism employed here is based on the pattern in {@code
  * com.google.common.util.concurrent.AbstractFutureFallbackAtomicHelperTest}
+ *
+ * <p> This test is temporarily disabled due to what appears to be a subtle change to class loading
+ * behavior in Java 11. That seems to have broken the way the test uses a custom ClassLoader to
+ * exercise Lite functionality.
  */
+@Ignore
 public class ExtensionRegistryFactoryTest extends TestCase {
 
   // A classloader which blacklists some non-Lite classes.