Tests: streamline more code in the Tools subdirectory

While writing Movemesh tests (in #1679) I realize some operations would gain to be streamlined in appropriate functions put in the test toolbox. For instance:

• Interpreting the CLI. It is done under the hood in TestNS::FixtureNS::Model, but we may want this outside of a model fixture.
• Defining a very basic Model. In the library Model is a CRTP used to define the model tailored to the actual problem studied, but in tests we may want to add absolutely nothing to such a basic model. Providing this bare bone model in tools would avoid repeating it in several tests.