Add ui test of missing comma between object entries

Author: dtolnay

tests/ui/missing_comma.rs

@@ -0,0 +1,5 @@
+use serde_json::json;
+
+fn main() {
+    json!({ "1": "" "2": "" });
+}

tests/ui/missing_comma.stderr

@@ -0,0 +1,10 @@
+error: expected type, found `""`
+ --> $DIR/missing_comma.rs:4:18
+  |
+4 |     json!({ "1": "" "2": "" });
+  |                - ^^ expected type
+  |                |
+  |                tried to parse a type due to this type ascription
+  |
+  = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
+  = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information