Rename arg-handler test class.

Prior to this commit, the arg_handler.py test class was named:
SingleProjectJiraPullArgumentHandlerTestCase.  This matched the
old name of arg_handler.py: single_project_jira_pull.py (catchy, I
know).

When single_project_jira_pull.py was renamed, the test class name
was neglected.  Since bugging-out will soon support exporting from
github sites, this name is misleading.

This commit fixes this by changing the class name to
ArgumentHandlerTestCase.