testsuite::ticket-421 - Minimal test against missing variable initializations
(ticket-421) run -> NEST exits if test fails  

This test simulates all nodes providing V_m for a short while and
checks that V_m remains constant. This is a minimal test against
missing variable initializations, cf ticket #421.

- Passing this test does not mean that all variables are properly initialized. It may just catch some cases bad cases.
- Simulator response to initialization errors is stochastic, so if variables are not initialized properly, this test may
fail in some runs and not in others.
- Neuron models that do not initialize V_m to steady state, must be added to the exclude_models list below!
- If this test fails, please see installcheck.log for which model the test failed.
The check if that model really initializes the membrane potential V_m to the steady-state value in absence of any input. If not, add the model to the exclude_models list below.

Hans Ekkehard Plesser, 2010-05-05