Keep tests in json format

So the results are typed

tests_dir/swap_fields.result

@@ -1,9 +1 @@
-id,name,chief,age
-0,jack,0,22
-1,carl,0,20
-2,john,1,30
-3,dean,1,33
-4,eve,0,25
-5,duncan,4,30
-6,paul,4,30
-7,alia,1,28
+{"content": [[1, "carl", 0, 20], [2, "john", 1, 30], [3, "dean", 1, 33], [5, "duncan", 4, 30], [0, "jack", 0, 22], [4, "eve", 0, 25], [6, "paul", 4, 30], [7, "alia", 1, 28]], "header": ["id", "name", "chief", "age"]}