Convert CLI chgrp help tests as unit tests
Convert TestDownload.testInvalidInput as unit test
Convert CLI group help test as unit tests
Convert CLI user help test as unit tests
Convert CLI script help tests as unit tests
Convert CLI sessions help tests as unit tests
Add support for OMERO_SESSION_DIR environement variable

With this commit, having OMERO_SESSION_DIR set should allow all CLI commands
to default to another sessions directory than the default one.
Suppress the session-dir argument help and expose the envvar
Fix extended help to use OMERO_SESSIONS_DIR
Add unit tests for sessions store directory