iotests: Drop TEST_DIR filter from _filter_nbd

Sockets should be placed into $SOCK_DIR instead of $TEST_DIR, so remove
the $TEST_DIR filter from _filter_nbd.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20191017133155.5327-24-mreitz@redhat.com
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>