pylibmount: remove unncessary subdirectory

Signed-off-by: Karel Zak <kzak@redhat.com>
pylibmount: import directly from pylibmount in tests

This change does not have any impact to in a standard way installed
libmount impact. It's simplification for in-tree tests.

Signed-off-by: Karel Zak <kzak@redhat.com>