selftest: Export DC conf path for special cases

selftest: Export DC conf path for special cases

In a few rare cases, a test needs to assert aspects both client-side and
server-side aspects. A typical example would be the audit logging, which
is exercising client-side behaviour, but also asserting the server-side
logging.

Usually this has involved a kludge in tests.py to either use
socket-wrapper explicitly, or hardcode in the server smb.conf path.

This patch exposes the existing SERVERCONFFILE env variable to the
tests. DC_SERVERCONFFILE has been added for 2 DC testenvs, where we need
the PDC's smb.conf.

The benefit of doing this way is the filepath/testenv-dependency logic
is all self-contained with the Perl code, and it doesn't bleed out into
tests.py as well.

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