Test pipelines with pipelines provided by fleet

[jsoriano]

Current pipeline tests install the pipelines included in packages and test them standalone using the simulate API.

In real scenarios Fleet also adds some processing, this is tested on system tests, but not on pipeline tests.

It would be good to somehow test pipelines with the processing added by Fleet, for scenarios that are difficult to test with system tests, such as the Windows integration.

This could be done by installing the whole package and still use the simulate API, instead of installing the pipelines only. Validation of received documents would need to be different in this mode because we wouldn't have the same level of control on the resulting documents. It would also add Kibana as dependency to pipeline tests, that currently depend on Elasticsearch only.