the `plugin_proxy_rewrite_args` method used for testing can cause unstable test cases

[Firstsawyou]

Issue description

The output data of the plugin_proxy_rewrite_args method in the apisix/t/lib/server.lua file is disordered, which will result in unstable test cases using this method. E.g:

--- request
GET /plugin_proxy_rewrite_args?name=james&age=36
--- response_body
uri: /plugin_proxy_rewrite_args
age2: 36
name2: james
--- no_error_log
[error]

--- request
GET /plugin_proxy_rewrite_args?name=james&age=36
--- response_body
uri: /plugin_proxy_rewrite_args
name2: james
age2: 36
--- no_error_log
[error]

Environment

• apisix version (cmd: apisix version):2.2
• OS: (cmd: uname -a): Linux
• OpenResty / Nginx version: (cmd: nginx -V or openresty -V): openresty/1.19.3.1