Add measurements to tests

[THuckemann]

We could use the dummy instruments to write some basic tests for the measurements. This should increase coverage by quite a bit and should not be to hard to implement. One could start based on the buffered_dummy_example, it should in principle work with all generic measurement scripts (with some small modifications).

We just have to keep in mind that the voltage parameter of the dmm only returns arrays atm, so we need to use the current param or adapt the voltage one. As the current returns random values it probably has to be adjusted to test break conditions.