Ava configuration should init endo as necessary

[gibson042]

(originally posted by @gibson042 in Agoric/agoric-sdk#8205 (review))

test-….js files in contract/test/ expect an import of @agoric/zoe/tools/prepare-test-env.js to initialize a Hardened JS environment (and possibly a vat-like one) via @agoric/zoe/tools/prepare-test-env.js importing @agoric/swingset-liveslots/tools/prepare-test-env.js, which imports @agoric/internal/src/install-ses-debug.js, which imports @endo/init/debug.js.

That's a brittle chain, and was temporarily broken during iteration on Agoric/agoric-sdk#8205 . Instead, this repository should adopt the pattern of that PR and init the expected environment in ava configuration.