software/erp5/test: fix ResourceWarning on python3
- use the requests.Session in a context manager so that resources are properly closed. - close the stdout of caucase process we want to keep silent See merge request nexedi/slapos!1075
- use the requests.Session in a context manager so that resources are properly closed. - close the stdout of caucase process we want to keep silent See merge request nexedi/slapos!1075