Disable test-660 for no-prebuild

Disable test-660 for no-prebuild since it does not generate app image to
load.

Test: testrunner.py --no-prebuild -t 660
Change-Id: I3ab2f96b7d3fbc994337c3a375663860da848591
diff --git a/test/knownfailures.json b/test/knownfailures.json
index 304b760..09e76fa 100644
--- a/test/knownfailures.json
+++ b/test/knownfailures.json
@@ -715,7 +715,8 @@
     },
     {
         "tests": "660-clinit",
-        "variant": "no-image | no-dex2oat",
-        "description": "Tests <clinit> for app images, which --no-image and --no-dex2oat do not create"
+        "variant": "no-image | no-dex2oat | no-prebuild",
+        "description": ["Tests <clinit> for app images, which --no-image, --no-prebuild and",
+                        "--no-dex2oat do not create"]
     }
 ]