Simplify application tests; remove unneeded mocking

This is a pretty simple tests-only change. After !197 (merged) Application class no longer does argv pre-parsing in __init__. One of the reasons for avoiding such work (in this case, accessing the external state of sys.argv) in __init__ is to avoid the need for mocking every time an instance of the class is created in tests. This allows some simplification of the Application tests.