Ignore false positive errorprone error UnusedMethod

Release-Notes: skip
Change-Id: I3aa95f4ca0efa985f65f9daaee88732391190de5

javatests/com/google/gerrit/server/update/context/RefUpdateContextTest.java

diff --git a/javatests/com/google/gerrit/server/update/context/RefUpdateContextTest.java b/javatests/com/google/gerrit/server/update/context/RefUpdateContextTest.java
index 55ec604..f2b481f 100644
--- a/javatests/com/google/gerrit/server/update/context/RefUpdateContextTest.java
+++ b/javatests/com/google/gerrit/server/update/context/RefUpdateContextTest.java
@@ -197,6 +197,7 @@
   }
 
   private static interface Callback {
+    @SuppressWarnings("unused")
     void callback();
   }
 }