Sanity tests

[AnHeuermann]

Add a bunch of sanity tests. Small models that test basic capabilities of the newBackend.

Those are easy to debug and can reveal issues.

[AnHeuermann]

A hello World test (der(x) = a *x) and a simple for-loop added in 4befe74.

[kabdelhak]

Test proposals:

• pure algebraic test (no states)
• linear loop
• nonlinear loop
• static index reduction (electrical circuit)
• dynamic index reduction (planar pendulum)