Reorganize trailing comma code a bit.

- Split out syntax error tests.
- Avoid redundant handling of empty literals.

test/language/list/trailing_comma.wren

@@ -1,12 +1,4 @@
-var list = [
-  "a",
-  "b",
-]
+var list = ["a", "b",]
 
 IO.print(list[0]) // expect: a
 IO.print(list[1]) // expect: b
-
-// Invalid syntax
-IO.print(new Fiber { Meta.eval("[,]")    }.try()) // expect: Could not compile source code.
-IO.print(new Fiber { Meta.eval("[1,,]")  }.try()) // expect: Could not compile source code.
-IO.print(new Fiber { Meta.eval("[1,,2]") }.try()) // expect: Could not compile source code.