Tests for BeanUtil class are missing

[tepi]

In flow-server module we have a helper class used when generating bean property sets, com.vaadin.flow.internal.BeanUtil. Currently we do not seem to have any tests for this helper class. It is somewhat tested indirectly via other tests but still, we should add independent unit tests for it.

[Paul-Hoke]

It looks like a lot of the units of work here are done in private methods, but nothing some reflection can't help us with. Picking this up.