Makefile: Update the targets for generating the API and running tests
Makefile: Update the targets for generating the API and running tests

* The `gen-api` and `gen-tests` targets generate both the Core and X-Pack APIs
* The `test-api` target runs the Core APIs by default, and the X-Pack APIs when the `flavor=xpack` variable is set

elastic/go-elasticsearch,elastic/go-elasticsearch,elastic/go-elasticsearch,elastic/go-elasticsearch