Build: Fix jarHell error I caused by last backport

I fixed this locally but didn't commit it.... Ooops. Shouldn't merge
late a night....

x-pack/qa/kerberos-tests/build.gradle

@@ -7,7 +7,7 @@ apply plugin: 'elasticsearch.standalone-rest-test'
 apply plugin: 'elasticsearch.rest-test'
 
 dependencies {
-    testCompile project(path: xpackModule('core'), configuration: 'runtime')
+    testCompile project(path: xpackModule('core'), configuration: 'shadow')
     testCompile project(path: xpackModule('core'), configuration: 'testArtifacts')
     testCompile project(path: xpackModule('security'), configuration: 'testArtifacts')
 }