signindex: add JSON check for entry.json

Ran this to generate/refresh the test index files:
`cd tests; ../fdroid update --nosign --pretty`

* converts config.py description to a single line, since the values are no
  longer stripped, so this give the same output.

closes #835

tests/repo/index-v1.json

@@ -1,6 +1,6 @@
 {
   "repo": {
-    "timestamp": 1502845383782,
+    "timestamp": 1676634233000,
     "version": 20002,
     "name": "My First F-Droid Repo Demo",
     "icon": "icon.png",
@@ -698,4 +698,4 @@
       }
     ]
   }
-}
+}