test: kill '"filename": "signature.asc"' from json output

This starts to appear with emacs24.4, so we can't easily have it in
our expected output.

test/test-lib.sh

@@ -690,6 +690,7 @@ notmuch_json_show_sanitize ()
     sed \
 	-e 's|"id": "[^"]*",|"id": "XXXXX",|g' \
 	-e 's|"Date": "Fri, 05 Jan 2001 [^"]*0000"|"Date": "GENERATED_DATE"|g' \
+	-e 's|"filename": "signature.asc",||g' \
 	-e 's|"filename": "/[^"]*",|"filename": "YYYYY",|g' \
 	-e 's|"timestamp": 97.......|"timestamp": 42|g'
 }