Make tests use GaugeUpdate::set_gauge_to_reference()

[luhuhis]

This is a follow up of #46

Tests that compare some observable to some value on some reference conf should now use GaugeUpdate::set_gauge_to_reference(). For example the ColorElectricCorrTest could be changed to use this, and many others.

Of course this does not apply to the tests that generate configurations, as there we need to compare to some reference conf that is stored in the repository.