And move emqx app back to root level
After realising the challenges of developing plugins in standalone
rebar projects. it is perhaps more smooth to keep using emqx as
a rebar dependency
* Wrapper proper test cases into common test cases
* Improve test cases for reason code module (#2789)
* Split 3 proper tests into 3 ct cases
* Improve test cases for client, rpc and request-response
* Add psk suites to increase coverage
* Add sys test cases
* Delete response_topic_prefix
The Response-Infomation in CONNACK was misinterpreated, it should
not be a broker's global config, instead, it should be per-client
or even per-session prefix/suffix mechanism
* Delete request-response code from emqx_client
* Add request-response test code
* meck as default dependency --- temp workaround for dep-vsn-check
Zaiming Shi