Added filesystem tests; fixed invalidation

Filesystem invalidation previously depended on the server notifying us
of a change; because this is an asynchronous process, the client would
return a stale filesystem snapshot if you asked for one immediately
after performing an operation. Now, it invalidates the snapshot after
every operation.

Added some more filesystem tests - also found another bug, that moving
items is not working properly.
Fix: moving items now works fine

A condition was reversed and so it did not allow you to move to valid
locations.
Added folder move test
Optimized file transfers and tests

File transfers were being rate limited because unused HTTP connections
were accidentally being left open - they are now closed. There are
theoretically more optimization possibilities by reusing connections,
but it is good enough right now.

File content comparison turned out to be extremely slow in tests. Now it
is fast and the file transfer tests are actually usable.
Removed obsolete comment