Fix int64/uint64 JSON parsing test

New libyang enforces that these 64bit integers are passed as strings.
That goes back to the JSON definition where these have to go as strings
so that they are not accidentally treated as floating point numbers.
That's JS heritage, you know.

Change-Id: Ia15c490069f345a7895851b7965b1344f4a355f4
diff --git a/tests/utils.cpp b/tests/utils.cpp
index 4b336b7..187cdf3 100644
--- a/tests/utils.cpp
+++ b/tests/utils.cpp
@@ -228,11 +228,11 @@
     "test-schema:int8": 8,
     "test-schema:int16": 300,
     "test-schema:int32": -300,
-    "test-schema:int64": -999999999999999,
+    "test-schema:int64": "-999999999999999",
     "test-schema:uint8": 8,
     "test-schema:uint16": 300,
     "test-schema:uint32": 300,
-    "test-schema:uint64": 999999999999999,
+    "test-schema:uint64": "999999999999999",
     "test-schema:boolean": true,
     "test-schema:string": "AHOJ",
     "test-schema:enum": "A",