Fix tests for save/load status state functionality

[samuelallan72]

The current tests for persistence (test_stored, test_stored_blank) pass, but they pass only because they reuse the same StatusPool class, and the statuses are persisted because they are class level variables. Not sure if this is a bug in the test harness (handling charm storage differently to prod), or the charm (running functionality or making assumptions that only hold true in certain prod environments).