Fix passing testImportMultiGroup integration test and remove xfail

- Modify get_object to pass {'omero.group': -1}
- Set arguments rather than session key to allow -g
Refactor set_conn_args() and move '--sudo' arguments to the tests
Pass omero.group to omero.sys.Principal when creating sudo session
Remove xfail mark from testImportAsRootMultiGroup
Rename test_login.py as test_sessions.py and remove duplicate register
Add eventContext test to testLoginAsRoot
Add eventContext test to testLoginAsGroupAdmin
Remove unused capsys fixture
Add integration tests for login in multi-group with/without sudo
Refactor connection arguments settings for Login tests
Store the new event context during session group for testing
Add integration test for session group command
Merge pull request #2620 from sbesson/12288_cli_import_group

Fix CLI import as with -g argument