Added test cases for the following APIs:
1. PUT /system/purgeaudit/{purge_id} Stop the specific purge audit log execution
2. GET /system/purgeaudit/{purge_id} Get purge job status
3. GET /system/purgeaudit/{purge_id}/log Get purge job log
4. PUT /system/purgeaudit/schedule Update purge job's schedule
5. POST /system/purgeaudit/schedule Create a purge job schedule
6. GET /system/purgeaudit/schedule Get purge's schedule
7. GET /system/purgeaudit Get purge job results.
Signed-off-by: Yang Jiao <jiaoya@vmware.com>
