refactor variable context:

1, variable context has two level, testset and testcase;
2, testset level variables can be used in whole test suite, while testcase level variables can only be used in testcase;
3, when variable binds with functions, the funtions will be called and the result will be set to the variable.

tests/data/demo_import_functions.yml

@@ -4,14 +4,13 @@
         - tests.data.custom_functions
     variable_binds:
         - TOKEN: debugtalk
-        - json: {}
-        - random: ${gen_random_string(5)}
-        - authorization: ${gen_md5($TOKEN, $json, $random)}
 
 - test:
     name: create user which does not exist
     variable_binds:
         - json: {"name": "user", "password": "123456"}
+        - random: ${gen_random_string(5)}
+        - authorization: ${gen_md5($TOKEN, $json, $random)}
     request:
         url: http://127.0.0.1:5000/api/users/1000
         method: POST
@@ -28,6 +27,8 @@
     name: create user which does not exist
     variable_binds:
         - json: {"name": "user", "password": "123456"}
+        - random: ${gen_random_string(5)}
+        - authorization: ${gen_md5($TOKEN, $json, $random)}
     request:
         url: http://127.0.0.1:5000/api/users/1000
         method: POST