s4: adding a DsdbTestCase class

[Nadezhda Ivanova]

Hi Jelmer and team,
In the testsuites in dsdb/tests there is a practise currently that each test
suite defines its own methods, even if they are the same as the ones used in
other testsuites, such as delete_force, get_domain_sid and a few others. To
avoid this, I suggest adding a DsdbTestCase class to samba.tests that
contains these methods, and all the testsuites that use them to inherit it,
as is the case with BlackboxTestCase and others. I have added the class
here:

http://gitweb.samba.org/?p=nivanova/samba.git;a=commit;h=4bf20e6948133c3a53b91da6c18bf597220082c5

And here is what acl and sec_descriptor look like now.
http://gitweb.samba.org/?p=nivanova/samba.git;a=commit;h=4f09b500b553c16a1d2263c9916c04343cde8522
http://gitweb.samba.org/?p=nivanova/samba.git;a=commit;h=a6986796fe5c248f642ddb6f855466aed4c03782

If it is OK with everyone, I would like to push this to master and change
the other testsuites in dsdb/tests to inherit DsdbTestCase too.

Regards,
Nadya