Extract systemtests into another package

[julienrbrt]

As discussed with @alpe, we should extract the systemtests framework in another package and leave our system tests under tests/
This allows us to release the framework without our tests, and as well make sure our tests aren't using internal systemtests logic.

I'd propose to put the systemtests package directly at the root. systemtests/go.mod and keep the tests/ package only for our tests.
(Eventually the same should be done done the integration test framework)