Add tests for module enable/disable

[andrewkroh]

Add tests for the Beats the support module enable and module disable commands. We test this from the Beats' system tests, but testing it here could add value since it will ensure the path.* configs are set correctly and that the permissions and file ownership are correct such that the Beat can rename the files.

See https://github.com/elastic/beats-tester/pull/87#issuecomment-416484678