Run system tests with clean data directories

Beats managed by the agent in system tests can store information in their data directories. This information persists between tests and may affect the result of later executions Tests should be executed on isolation, with clean data directories.

In https://github.com/elastic/integrations/pull/1145 I found that after running system tests several times, one of the data streams started to fail frequently because it couldn't find documents in the data stream. I saw that the filebeat registry contained lots of entries, many of them for the file read by the data stream. Not 100% sure, but I think this might be the cause of the failures, filebeat may not be reading the file because it was already read in the past to a similar offset.