Minor tweak to packaging tests.

Minor tweak to packaging tests.

When an option is changed on a command object, calling ensure_finalized
for a second time will not run finalize_options again, because
ensure_finalized is a no-op the second time.  By resetting the finalized
attribute, we can be sure that whatever computation takes place in
finalize_options will happen again.

(In test_command_clean, I removed two lines that were a no-op.)