Blacklist test 596/597 for no-prebuild

Configuration doesn't generate app images.

Bug: 149098478
Test: art/test/testrunner/run_build_test_target.py -j110 art-no-prebuild
Change-Id: I672e83f8ee0194808ec77ab05604fe9dbca7aeff
diff --git a/test/knownfailures.json b/test/knownfailures.json
index 3eecf7a..a55bbd5 100644
--- a/test/knownfailures.json
+++ b/test/knownfailures.json
@@ -305,7 +305,7 @@
     {
         "tests": ["596-app-images", "597-app-images-same-classloader"],
         "description": "no image generated in configuration",
-        "variant": "no-image"
+        "variant": "no-image | no-prebuild"
     },
     {
         "tests": ["596-app-images", "597-app-images-same-classloader"],