Make tests of different filesystems actually test the different filesystems

Doing += to an attribute of 'Building' accumulates across loop iterations because it's a global object. This is probably not what was meant to happen by looping through filesystems to test.

Fortunately the only `#IFDEF`s so far are for `NODEFS`, and since `NODEFS` happens to be second in the list we always test both filesystems.

A bug waiting to happen I think though.