Disable 691-hiddenapi-proxy on for redefine tests

The test depends on appending a custom dex to boot classpath
which is not compatible with the redefine test environment.

Test: art/test.py -r -b -t 691-hiddenapi-proxy
Change-Id: Ib0cddf3a5337fcfa85bb83cd42c0f201734f4de1
diff --git a/test/knownfailures.json b/test/knownfailures.json
index e4b3f59..927817a 100644
--- a/test/knownfailures.json
+++ b/test/knownfailures.json
@@ -557,6 +557,7 @@
             "647-jni-get-field-id",
             "674-hiddenapi",
             "690-hiddenapi-same-name-methods",
+            "691-hiddenapi-proxy",
             "944-transform-classloaders",
             "999-redefine-hiddenapi"
         ],