Title: #748: TESTS: Don't fail when trying to create an OU that already exists
If a fixture that creates an OU was used twice, the second invocation would
fail, because the OU already exists. This patch ignores the already exists
error when adding an OU.
To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/748/head:pr748
git checkout pr748