Use Test::Nginx::Socket::Lua instead of Test::Nginx::Socket in the test suite

Please use Test;:Nginx::Socket::Lua instead of Test::Nginx::Socket so that we can run the test suite with lua-resty-core easily by specifying the system environment variable

``` bash
export TEST_NGINX_INIT_BY_LUA="package.path = '/path/to/lua-resty-core/lib/?.lua;' .. (package.path or '') require 'resty.core'"
```
