s4:drs:test:getncchanges: add a timeout failure

s4:drs:test:getncchanges: add a timeout failure

In the next commit we are going to add tests in which the client
modifies the highwatermark in a way that resets replication (on Samba
only). After that we'll fix it.

If we leave the test in an eternal loop, the commit history will not
be bisectable, so we are temporarily going to turn long waits into
failures.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15701

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jennifer Sutton <josutton@catalyst.net.nz>