[porousmediumflow][2pncmin] Use correct time step size for problem file

In the 2pncmin test the time step size is used in the problem for calculating the amount of precipitating salt per time. For this the time step is set in the main file at the beginning of each new time step. For this case this works so far.

However, @Simon Grether pointed out that for simulations where the newton solver does not converge and hence reduces the time step, this is not correctly implemented. This modification of the time step is not accounted for in the problem file, when just setting the time step once in the time loop. Hence this would be nice to adapt already in the 2pncmin test, such that if this test case is copied and gets modified there is one possible error source less.