Add a test suite for `flow_stats` endpoints

[viniarck]

This is for adding a test suite for flow_stats and covering the following endpoints:

• GET v1/flow/stats
• GET v1/packet_count/<flow_id>
• GET v1/bytes_count/<flow_id>
• GET v1/packet_count/per_flow/<dpid>
• GET v1/bytes_count/per_flow/<dpid>

Each endpoint should have its own test case, exercising what's expected to be covered to ensure that the functionality works.

Here's the endpoint OpenAPI spec documentation.

You can use the ring topology to facilitate. You'll need to also have flow provisioned, you can either leverage existing expected flows such as from of_lldp or provision new ones with mef_eline or flow_manager.

[viniarck]

Tests for kytos_stats which is flow_stats successor has landed on https://github.com/kytos-ng/kytos-end-to-end-tests/pull/253