[Feature]: Proper tests for Equilibrium Pressure Outlet

[PhiSpel]

Contact Details

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Is your proposal related to a problem?

Currently, we do not have a test for EquilibriumOutletP with low enough tolerances.

Describe the solution you'd like

Lower the tolerances by giving the simulation time to reach steady state

Describe alternatives you've considered

Do fewer time steps to a point where an analytical solution should be acchieved

Additional context

1. test_equilibrium_outlet_p_algorithm does only one timestep, but with a high relative tolerance
2. test_equilibrium_pressure_outlet does 30 timesteps, but with an obstacle which itself induces oscillations