selftest: mark the samba4.blackbox.samba_tool_demote test flakey.

This is causing frequent autobuild failures lately like this:

[1695(11293)/1735 at 1h54m54s] samba4.blackbox.samba_tool_demote(promoted_dc)
Using localdc as partner server for the demotion
Deactivating inbound replication
Asking partner server localdc to synchronize from us
Error while demoting, re-enabling inbound replication
ERROR(<class 'samba.drs_utils.drsException'>): Error while sending a DsReplicaSync for partion CN=Schema,CN=Configuration,DC=samba,DC=example,DC=com - drsException: DsReplicaSync failed (8442, 'WERR_DS_DRA_INTERNAL_ERROR')
  File "bin/python/samba/netcmd/domain.py", line 712, in run
    sendDsReplicaSync(drsuapiBind, drsuapi_handle, ntds_guid, str(part), drsuapi.DRSUAPI_DRS_WRIT_REP)
  File "bin/python/samba/drs_utils.py", line 83, in sendDsReplicaSync
    raise drsException("DsReplicaSync failed %s" % estr)
UNEXPECTED(failure): samba4.blackbox.samba_tool_demote(promoted_dc).demote(promoted_dc)
REASON: Exception: Exception: No reason specified

FAILED (1 failures, 0 errors and 0 unexpected successes in 0 testsuites)

Also seen from time to time in intermittent autobuild failure checks:

https://git.samba.org/autobuild.flakey/2012-07-10-0942/samba.stdout
https://git.samba.org/autobuild.flakey/2012-07-15-1142/samba.stdout
https://git.samba.org/autobuild.flakey/2012-08-05-1944/samba.stdout
https://git.samba.org/autobuild.flakey/2013-01-12-0931/samba.stdout
https://git.samba.org/autobuild.flakey/2013-01-16-1936/samba.stdout
https://git.samba.org/autobuild.flakey/2013-01-21-2330/samba.stdout
https://git.samba.org/autobuild.flakey/2013-01-26-0528/samba.stdout
...
https://git.samba.org/autobuild.flakey/2014-11-13-2305/samba.stdout
https://git.samba.org/autobuild.flakey/2014-11-15-1710/samba.stdout
https://git.samba.org/autobuild.flakey/2014-11-21-0505/samba.stdout
https://git.samba.org/autobuild.flakey/2014-11-27-0810/samba.stdout
https://git.samba.org/autobuild.flakey/2014-11-30-0215/samba.stdout
https://git.samba.org/autobuild.flakey/2014-12-04-1409/samba.stdout
https://git.samba.org/autobuild.flakey/2015-01-14-2008/samba.stdout
https://git.samba.org/autobuild.flakey/2015-01-31-2010/samba.stdout
https://git.samba.org/autobuild.flakey/2015-03-19-1415/samba.stdout

Possibly related to the flakeyness of the tombstone_reanimation
test and the drs.delete_object tests.

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>

selftest/flapping

@@ -27,3 +27,4 @@
 ^samba3.raw.samba3closeerr.samba3closeerr\(ad_dc\) # Seems to flap - succeeds on sn-devel, fails on Fedora 16
 ^samba4.tombstone_reanimation.python # flakey on sn-devel
 ^samba4.drs.delete_object.python # flakey on sn-devel
+^samba4.blackbox.samba_tool_demote # flakey on sn-devel