Moved tests around.
Moved tests around.

The test cases that verify that the AtomEventStream correctly yields its
events is a new group of tests, so there's no reason they should remove
the AppendAsync tests from their private helper classes. This follows
Uncle Bob/Clean Code's concept of code being readable as a newspaper
article: from the top containing general overview, to increasin levels of
detail.

GreanTech/AtomEventStore,GreanTech/AtomEventStore