Gradually move E2E specs from 'test' to 'it'

[subotic]

Currently, the tests under test can be grouped into three different types:

• unit tests, i.e., tests which don't need anything else but webapi
• extended "unit tests", i.e., tests which only require GraphDB
• E2E tests, which require a running webapi server and GraphDB

As soon as #1470 is resolved, I would like to make the extended "unit tests" more integrated, i.e., have them running with an embedded GraphDB.

This would simplify and speed up local development since nothing else would need to run on the side to be able to execute these tests.

This would mean, that the E2E tests would need to be moved to the it folder tree, where they actually make more sense. Also, there are some performance tests using Gatling, which could be moved into an entirely new perftest tree.

At the same time, I would like to run the integration tests in the it tree in a more "real-life" way. They should run against an already running knora-stack, i.e., the tests themselves would not start the webapi service, but only send HTTP requests. This would allow us to run these tests not only locally but also against a real knora-stack deployment.

Since there is a number of methods used both in the test and it tree, it will probably make sense to move them to a knora-test library, which could then be imported by both test and it.

related: #1501