selftest: Print error message when smbd does not have ADS support

When smbd cannot be compiled with ADS support, setting up the s3member
environment fails with:

samba: using 'standard' process model
Samba can't provide environment 's3member' at /test/samba/selftest/target/Samba.pm line 44.
Can't use string ("UNKNOWN") as a HASH ref while "strict refs" in use at /test/samba/selftest/selftest.pl line 852.
samba: EOF on stdin - terminating

Add an explicit error message for the missing ADS support to make this
easier to debug and also avoid the warning about the hash reference:

samba: using 'standard' process model
Samba can't provide environment 's3member' at /test/samba/selftest/target/Samba.pm line 44.
Unable to setup environment s3member at /test/samba/selftest/selftest.pl line 851.
smbd does not have ADS support
samba: EOF on stdin - terminating

Signed-off-by: Christof Schmitt <christof.schmitt@us.ibm.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Sat Jul 27 08:31:14 CEST 2013 on sn-devel-104