feat: testsuite config get variables by call function

examples/postman_echo/debugtalk.py

@@ -7,3 +7,9 @@ def get_httprunner_version():
 
 def sum_two(m, n):
     return m + n
+
+
+def get_variables():
+    return {
+        "foo1": "session_bar1"
+    }

examples/postman_echo/request_methods/demo_testsuite.yml

@@ -1,6 +1,6 @@
 config:
     name: "demo testsuite"
-#    variables: ${get_variable()}
+    variables: ${get_variables()}
 
 testcases:
 -