Add missing unit tests

[Roman2K]

Parts that most necessitate unit tests should already be covered. Add tests for those uncovered areas. Notably:

• stores.Mem
• argparse.ArgFilter
• argparse.ArgOr
• procs.Chain: test err in ProcessFinal()
• procs.Chain: test err in ProcessEnd()
• procs/backlog
• procs/concur_test: avoid durations to avoid race conditions