Refactor JSON encoding and decoding tests for improved clarity and error handling

Author: bobzhang

json/json_encode_decode_test.mbt

@@ -52,7 +52,12 @@ pub impl Show for DecodeError with output(self, logger) {
 ///|
 fn of_json(jv : Json) -> AllThree!DecodeError {
   match jv {
-    { "ints": Array(ints), "floats": Array(floats), "strings": Array(strings) } => {
+    {
+      "ints": Array(ints),
+      "floats": Array(floats),
+      "strings": Array(strings),
+      ..
+    } => {
       let ints_result = []
       let floats_result = []
       let strings_result = []

json/json_test.mbt

@@ -274,7 +274,7 @@ test "stringify" {
   // we do come across issues like ParseError not unified with String
   let newjson = @json.parse!(json.stringify())
   match json {
-    { "key": [_, _, _, _, { "value": Number(i) }, ..] } =>
+    { "key": [_, _, _, _, { "value": Number(i), .. }, ..], .. } =>
       inspect!(i, content="100")
     _ => fail!("Failed to match the JSON")
   }