Introduce tests to encode and decode json

upb/json_encode.c

diff --git a/upb/json_encode.c b/upb/json_encode.c
index 848290f..a247078 100644
--- a/upb/json_encode.c
+++ b/upb/json_encode.c
@@ -163,7 +163,7 @@
    *   Processing Calendar Dates," Communications of the Association of
    *   Computing Machines, vol. 11 (1968), p. 657.  */
   seconds += 62135596800; // Ensure seconds is positive. 
-  L = seconds / 86400 - 719162 + 68569 + 2440588
+  L = (int)(seconds / 86400) - 719162 + 68569 + 2440588;
   N = 4 * L / 146097;
   L = L - (146097 * N + 3) / 4;
   I = 4000 * (L + 1) / 1461001;