Fixed tests for flavor swap extension after merging in core API

flavor_swap extension was merged into core API but tests were not
updated accordinly so this functionality was not tested at all.
This commit adds proper tests for this extension

Related to blueprint nova-v3-api

Change-Id: If5f464302f2efd51d6c48d9c28baa6e0922bfa2c
Merge "Fixed tests for flavor swap extension after merging in core API"