tests(pdk) test for kong.request.get_path_with_query on error requests (#3868)
tests(pdk) test for kong.request.get_path_with_query on error requests (#3868)

Related with https://github.com/Kong/kong-plugin-zipkin/pull/24

Explanation:

When kong receives an invalid request, it activates its [internal `kong_error_handler` location](https://github.com/Kong/kong/blob/master/kong/templates/nginx_kong.lua#L164):

```
    error_page 400 404 408 411 412 413 414 417 494 /kong_error_handler;
    error_page 500 502 503 504 /kong_error_handler;
    
    ...
    
    location = /kong_error_handler {
        internal;
        uninitialized_variable_warn off;
        content_by_lua_block {
            Kong.handle_error()
        }
        header_filter_by_lua_block {
            Kong.header_filter()
        }
        body_filter_by_lua_block {
            Kong.body_filter()
        }
        log_by_lua_block {
            Kong.log()
        }
    }
```

This code will activate plugins which have header filters, body filters or log phases. On these cases, `ngx.var.request_uri` can indeed be `nil` (on invalid http requests for example), so the test makes sure that even in those cases `kong.request.get_path_with_query` is an empty string (and not nil).

Notes:
* On this test, the error which triggers the error handler is 400, since sending `invalid http request` over a socket is not a valid http request.
* It was not possible to send a non-http request directly from the `request` section of `test-nginx` so instead we made nginx "call himself" via a socket.
Kong/kong,Kong/kong,Kong/kong,Mashape/kong