Clean tests directory

[awegsche]

Issue

The current layout of the tests directory is overly complicated and has confusing folder names (e.g. outputs inside of an inputs root folder).
The complexity of this structure is expected to increase over time.

Suggested solution:

1. move test and verification data next to the tests, where applicable (see 2.)
   Pros:

   • easier to write new tests
   • easier to understand how a test works and where its inputs come from

   Cons:

   • mixes code with assets

2. for global inputs (models, .linx/y files that are used by different test modules), a global testassets folder makes sense. naming has to be chosen carefully.

[mihofer]

Create global test constant for input dir, to replace in CRDT and other tests the Path(./).parent.parent