improve/audit liveslots virtual-collections tests

[warner]

What is the Problem Being Solved?

One consequence of bug #8756 was that using a collection after doing collection.clear() did not keep working correctly: .has(oldkey) reported true, .init(oldkey, newvalue) failed.

In the PR for the fix (#9961), I updated the tests to exercise the cases that were failing before. But the fact that the existing tests did not catch this means that we should probably take a critical look at those tests, and make sure they're covering everything we can think of.

The tests live in https://github.com/Agoric/agoric-sdk/blob/master/packages/swingset-liveslots/test/collections.test.js

[warner]

#10007