improve tests by using a single server in conftest.py

[Shivashis Padhi]

As of now, for each test unit a server starts during setup(). If it could be done as one section in conftest.py, like in mswms, it'd be more efficient.

[Shivashis Padhi]

https://bitbucket.org/wxmetvis/mss/pull-requests/704/refactor-tests-for-mscolab-start-server/diff

fixes this issue.