tests: Move repeated code into a helper function

tests: Move repeated code into a helper function

Several tests hang all the objects they create off a unique OU.
Having a common OU makes cleanup easier, and having a unique OU (i.e.
adding some randomness) helps protect against one-off test failures
(Replication between testenvs is happening in the background.
Occasionally, when a test finishes on one testenv and moves onto the
next testenv, that testenv may have received the replicated test
objects from the first testenv, but has not received their deletion
yet).

Rather than copy-n-pasting this code yet again, split it out into a
helper function.

Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>