make variables in testcase template compatible with mapping style:

new mapping style:
"variables": {
    "a": 1,
    "b": 2
}

Also, the former list style is still valid:
"variables": [
    {"a": 1},
    {"b": 2}
]

tests/data/demo_binds.yml

@@ -1,10 +1,10 @@
 bind_variables:
     variables:
-        - TOKEN: "debugtalk"
-        - token: $TOKEN
+        TOKEN: "debugtalk"
+        token: $TOKEN
 
 builtin_functions:
     variables:
-        - length: ${len(debugtalk)}
-        - smallest: ${min(2, 3, 8)}
-        - largest: ${max(2, 3, 8)}
+        length: ${len(debugtalk)}
+        smallest: ${min(2, 3, 8)}
+        largest: ${max(2, 3, 8)}