Rename core e2e tests

[Kr0nox]

The "e2e tests in the core" should be renamed to avoid confusion and name collisions with test e2e test, that run through the entire jplag process (inclusing jar and report viewer)

This is not a breaking change, as we do not publish that package.

This was first discussed in #2620

[tsaglam]

Maybe a more fitting name would be core regression tests or just regression tests, as they were originally meant to assess whether the performance (as in precision, not runtime) of JPlag changes.

[Kr0nox]

Package name and module should be named regression-test