test_2pncminni_salinization_tpfa questionable and (sporadically failing) test

The test test_2pncminni_salinization_tpfa supposedly simulates an evaporation scenario that should lead to salt precipitation. However during the runtime of the test nothing actually precipitates. Which raises the question: Does this actually test anything that is not tested in the 2pnc tests already?

This also currently leads to the test failing with certain configurations (see BuildBot) because no zero threshold is set for the precipitated volume fraction of NaCl. (Zero thresholds are always necessary if a parameter is exact zero everywhere because then there is no way to determine a reliable epsilon for floating point comparisons automatically.) This is also why this is marked for milestone 3.2.

I suggest to fix the test to actually test "salinization" as the name suggests.