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 …