(maint) Fix face specs to not pollute global state.

The face_collection_base spec attempts to restore global state stored in
Puppet::Interface::FaceCollection so that it doesn't interfere with
other specs that may use faces.

It fails to clear any files required by the autoloader, but clears the
@faces member. The result is that if a face is referenced again from
another spec, it doesn't get defined and added to @faces and cannot be
found.

The fix is to clear the state before and after each example.

The problem can be observed with the following:
rspec spec/unit/interface/face_collection_spec.rb spec/unit/application/face_base_spec.rb

Additionally, the indirection_base spec registers a face but does not remove the face
after the examples have fun. The fix is to remove the face from the FaceCollection after
all examples.