Tests lsa.String: add String constructor, str and repr

Tests for the String constructor, str and repr methods added to
the samba.dcerpc.lsa.String python object

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
