Core test should not be dependant in plugins

Core test should not be dependant on plugins to work. This days core test need

```

gem "logstash-filter-clone"
gem "logstash-filter-mutate"
gem "logstash-filter-multiline"
gem "logstash-input-generator"
gem "logstash-input-stdin"
gem "logstash-input-tcp"
gem "logstash-output-stdout"
```

to work, but for this test to be actually validating the pipeline behaviour it should be using mocks or internal dummy filters created for this special purpose. Otherwise when a plugin has a blocker update, test in core are not working as expected.