Add tests checking instance of `Backend` when progressbar is supposed to be displayed

[marinegor]

Piece of code:

mdanalysis/package/MDAnalysis/analysis/base.py

Lines 770 to 774 in f6672ff

	if ((progressbar_kwargs or verbose) and
	not (backend == "serial" or
	issubclass(backend, BackendSerial) or
	isinstance(backend, BackendSerial))):
	raise ValueError("Can not display progressbar with non-serial backend")

Currently tests don't cover that.