Tests for 'samba-tool user create' with RFC2307 attributes

Tests for 'samba-tool user create' with RFC2307 attributes

Check if attributes are correctly set and read from SamDB
Test automatic creation of attributes from getpwent (NSS)
Check if overriding NSS attributes works

getpwent will be skipped if the current UID of the user running the
tests has no passwd entry (getpwuid(geteuid())).

If a user with the name of the current UID already exists in the
directory, the getpwent test will fail. If that should happen, the
test would need to be updated to use a nonexistent UID that is
visible to the Python 'pwd' module.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>