Fix changes broken by renaming to ExceptionDummyBackend in DeviceAccess.

The default value was properly propagated but not visible to the test until a value was sent through the return channel.

This can be overridden by a newly introduced boolean argument. Including the return channel was causing problems e.g. when propagating the DataValidity flag.

need to wait until all mainLoops() are entered, so the initial value propagation is done. Otherwise during initial value propagation values could be read from hardware which might already trigger the exception state.

one initial value was not properly waited for, and one inherent race condition is now basically ignored (with comment)

this was violating thread safety of accessors, since the mainLoopWrapper() was reading initial values at the same time as the test thread was reading the same accessors.