selftest: use server_maxtime = 10800 by default

selftest: use server_maxtime = 10800 by default

With something like this:

samba.stdout:
[1855(18118)/1864 at 2h37m4s] samba4.krb5.kdc with account ALLOWED permission to replicate to an RODC(fl2008r2dc)
[1856(18378)/1864 at 2h37m57s] samba4.blackbox.dbcheck(ad_dc_ntvfs)
ERROR: Testsuite[samba4.blackbox.dbcheck(ad_dc_ntvfs)]
REASON: unable to set up environment ad_dc_ntvfs:local - exiting

samba.stderr:
samba: maximum runtime exceeded - terminating at 1435553350, current ts: 1435553350

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>