Allow to write tests with pressure devices

[t-b]

We don't yet have tests for the pressure control code, the line coverage for MIES_PressureControl.ipf is therefore only 24% 1. As recent issues have shown #2124, and also the additional fix required for #2137 (comment), this ain't good.

For being able to write tests using pressure control we would need a NI USB 6001 at all CI machines with hardware attached. If something has to be bought we might also buy three to prepare for a future separate machine for ITC1600 and NI.