Unit tests and functional tests for the solvers

[mvrahden]

Is your feature request related to a problem? Please describe.
The current implementation only tests the regular usage case once. There are no unit tests including spies and stubs to verify the right solver behavior over time.
As an example, see the recurrent-js implementation with currently ~170 individual test cases.
Also there could be a test case for each solver, that tests the prediction improvement over time.

Describe the solution you'd like
none

Describe alternatives you've considered
none

Additional context
none