Restore ldap.sort test of embedded NULLs

[Noel Power]

Hi,

To get the python3 port over the line I had to disable part of the tests 
performed by the 'samba4.ldap.sort.python'

The disabled part of the tests were to do with sorting attributes that 
had embedded NULLs in the values. For more technical detail about why I 
needed to disable them please have a look at the comments in 
https://gitlab.com/samba-team/devel/samba/commit/34ca15fb042e42773854c093ad9f1e67696c90ac

At the very least it seemed we should be able to get coverage with a 
python2 test (see 1st patch in series attached) And additionally I hoped 
that we could avoid the problematic python3 issue by using test data 
from files (see 2nd patch)

Please have a look at the attached patches (I propose we squash them but 
leaving them separate just incase there is more to do with the second) 
In otherwords at the very least I would like to submit the first patch 
and if there is agreement instead submit the 2 patches (squashed)

CI running https://gitlab.com/samba-team/devel/samba/pipelines/39709623 
(note: there are (well at the moment at least one) additional patches in 
this CI run)

Noel

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