$ rebar3 compile

# run mock server
$ cd mock_server
$ erl
1> c(simple_http_server).
{ok, simple_http_server}
2> simple_http_server:start(). %% http server on port 8080 starting up.
{ok, <PID>}

# run gun app
$ rebar3 shell
....
1> gun_test:start().
start supervisor
{ok,<0.141.0>}
2> gun_test:get().
get_request
ok
get_request Pid <0.147.0>
gun_up
gun_up_done
fin <<"{\"hello\" : \"ok\"}\n">>