[test][richards] the evaporation test does not actually calculate diffusion

The richards evaporation tests enable water diffusion in air. However, the BalanceEquationOptions are never changed, resulting in a diffusive flux of 0, because the mainComponentIsBalanced is set to true as a default and not overwritten (although there is a nice hint that for richards this should be changed). I tried to change the property for the test, however then convergence got very bad.

also maybe there should be some help that in case water diffusion is enabled, the BalanceEquationOptions are changed automatically?