Harden: test objects with getters/setters

[kumavis]

testing objects with getters + setters doesnt seem to be part of the test suite.

was experimenting with alternate implementation and all tests passed for my code that incorrectly handled getters and setters

[Tartuffo]

@kriskowal This does not have an area label that is covered by our weekly tech / planning meetings. Can you assign the proper label? We cover: agd, agoric-cosmos, amm, core economy, cosmic-swingset, endo, getrun, governance, installation-bundling, metering, run-protocol, staking, swingset, swingset-runner, token economy, wallet, zoe contract,