Split integration tests

[MrAlias]

• Ease replacement of the BATS tooling by allowing Go definitions of validation to be declared where the tests are run. Do not require the auto package to import all the e2e packages which is an import cycle.
• Structure testing in a way that will work with custom probes. When we accept custom probes, they will want to be able to include integration tests just like the ones we run. This restructuring will allow us to export some version of the e2e package to facilitate this.