s3/torture: local_rbtree: avoid birthday collisions

s3/torture: local_rbtree: avoid birthday collisions

We insert 999 keys, but if two of them happen to be the same, we
don't find 1000 nodes, and that is upsetting for CI:

[130(1421)/303 at 4m46s] samba3.smbtorture_s3.LOCAL-RBTREE
UNEXPECTED(failure): samba3.smbtorture_s3.LOCAL-RBTREE.smbtorture(none)
REASON: Exception: Exception: using seed 1716333987
host=foo share=bar user= myname=runner-jlguopmm-project-6378020-concurrent-0
Running LOCAL-RBTREE
run_local_rbtree: read1: 999 999, NT_STATUS_OK
run_local_rbtree: delete: 999 999, NT_STATUS_OK
run_local_rbtree: read2: 0 0, NT_STATUS_OK
TEST LOCAL-RBTREE FAILED!
LOCAL-RBTREE took 0.002706 secs

This has been flapping very occasionally for a long time:
https://lists.samba.org/archive/samba-technical/2016-March/112861.html

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>