Add format output for test print (#2076)

* Add formatted output for the test print

src/emqx_cli.erl

@@ -17,16 +17,17 @@
 -export([print/1, print/2, usage/1, usage/2]).
 
 print(Msg) ->
-    io:format(Msg).
+    io:format(Msg), lists:flatten(io_lib:format("~p", [Msg])).
 
 print(Format, Args) ->
-    io:format(Format, Args).
+    io:format(Format, Args), lists:flatten(io_lib:format(Format, Args)).
 
 usage(CmdList) ->
-    lists:foreach(
+    lists:map(
       fun({Cmd, Descr}) ->
-        io:format("~-48s# ~s~n", [Cmd, Descr])
+          io:format("~-48s# ~s~n", [Cmd, Descr]),
+          lists:flatten(io_lib:format("~-48s# ~s~n", [Cmd, Descr]))
       end, CmdList).
 
 usage(Format, Args) ->
-    usage([{Format, Args}]).
+    usage([{Format, Args}]).