Prior to this change, plugin config files are only allowed
to be placed in the collective config dir etc/plugins.
In order to support external plugin's drop-in deployment,
this commit made emqx_plugins module to read conf file
in application's etc dir
* 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