Add test folder

Status Quo

Currently, in vf we have three types of tests, which are a bit spread over the project:

• unit-tests in src/ next to the production code
• regression-tests in regression-tests/
• numerical tests in apps/gpu/numerical-tests

Proposal

Create a new structure:

• tests/
  • unit-tests/
  • regression-tests/
  • numerical-tests/

This keeps all tests in one place. Also other people, who are not so familiar with VF will find the tests quickly. Having a test/ folder is very common.

In the past we had the unit-tests next to the production code to have this close to each other. However, this comes with some drawback: We have to exclude the tests file from the production binaries. Because of this, currently it's not allowed to have Testor `Mock` in the name of a file because of this.