tests: Add a test case for msDS-PasswordReversibleEncryptionEnabled

tests: Add a test case for msDS-PasswordReversibleEncryptionEnabled

Add a test for the 'msDS-PasswordReversibleEncryptionEnabled' attribute
on the PSO. The Effective-PasswordReversibleEncryptionEnabled is
based on the PSO setting (if one applies) or else the
DOMAIN_PASSWORD_STORE_CLEARTEXT bit for the domain's pwdProperties.
This indicates whether the user's cleartext password is to be stored
in the supplementalCredentials attribute (as 'Primary:CLEARTEXT').

The password_hash tests already text the cleartext behaviour, so I've
added an additional test case for PSOs. Note that supplementary-
credential information is not returned over LDAP (the password_hash
test uses a local LDB connection), so it made more sense to extend
the password_hash tests than to check this behaviour as part of the
PSO tests (i.e. rather than in password_settings.py).

Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>