ExtractResourceWithChangesSCM now needs to have a descriptor.

Otherwise you get:
java.lang.AssertionError: class org.jvnet.hudson.test.ExtractResourceWithChangesSCM is missing its descriptor
	at jenkins.model.Jenkins.getDescriptorOrDie(Jenkins.java:1166)
	at hudson.scm.SCM.getDescriptor(SCM.java:654)
	at hudson.scm.AutoBrowserHolder.get(AutoBrowserHolder.java:54)
	at hudson.scm.SCM.getEffectiveBrowser(SCM.java:136)
	at hudson.scm.ChangeLogSet.browserFromBuild(ChangeLogSet.java:82)
	at hudson.scm.ChangeLogSet.<init>(ChangeLogSet.java:76)
	at org.jvnet.hudson.test.ExtractChangeLogSet.<init>(ExtractChangeLogSet.java:41)
	at org.jvnet.hudson.test.ExtractChangeLogParser.parse(ExtractChangeLogParser.java:80)
	at org.jvnet.hudson.test.ExtractChangeLogParser.parse(ExtractChangeLogParser.java:52)
	at org.jvnet.hudson.test.ExtractChangeLogParser.parse(ExtractChangeLogParser.java:46)
	at hudson.model.AbstractBuild.calcChangeSet(AbstractBuild.java:894)
	at hudson.model.AbstractBuild.access$600(AbstractBuild.java:104)
	at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:634)
	at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
	at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:528)
	at hudson.model.Run.execute(Run.java:1745)
	at …

test/src/main/java/org/jvnet/hudson/test/ExtractResourceWithChangesSCM.java

@@ -31,6 +31,7 @@ import hudson.model.BuildListener;
 import hudson.scm.ChangeLogParser;
 import hudson.scm.NullSCM;
 import hudson.scm.SCM;
+import hudson.scm.SCMDescriptor;
 
 import java.io.File;
 import java.io.FileOutputStream;
@@ -139,4 +140,13 @@ public class ExtractResourceWithChangesSCM extends NullSCM {
      * Don't write 'this', so that subtypes can be implemented as anonymous class.
      */
     private Object writeReplace() { return new Object(); }
+
+    @Override public SCMDescriptor<?> getDescriptor() {
+        return new SCMDescriptor<ExtractResourceWithChangesSCM>(ExtractResourceWithChangesSCM.class, null) {
+            @Override public String getDisplayName() {
+                return "ExtractResourceWithChangesSCM";
+            }
+        };
+    }
+
 }