Move mock_space_spec specs into space_spec.rb

Not sure how we wound up with two separate files, anyway.
Remove unnecessary collection clearing.

Since we now use a new space instance per example,
we don't need to clear its collections when resetting.
Before this was necessary because we kept a space instance
that we would keep using for the lifetime of the process.

Constant mutators weren't being reset idempotently, so I
had to tweak them a bit.