Fix some spurious test failures that get reported if some of the "before" actions of a test fail. Specifically, calling

persistence.shutdown()

would throw a

Uncaught TypeError: Cannot read properties of undefined

error if the "before" step that initialized the persistence variable failed before setting it.

Although this isn't a problem, it adds noise to a test failure that may already be difficult enough to debug.

The fix is simple: just conditionally call the method only if the object is defined; that is, change it to

persistence?.shutdown()