fix(async-flow): fix endowment equate bug

[erights]

closes: #XXXX
refs: #9722

Extracted from #9722 into an independently mergeable PR

Description

The failure at https://github.com/Agoric/agoric-sdk/pull/9719/files#diff-cbd091458bbd51ddc6640a3783cdb340d7f2588cb7ff27b330ba228cfbd0e18dR64-R87 revealed a bug introduced to async-flow when adding support for endowments. Because of the so-called "unwrapping" of some guests, there can be two guests corresponding to one host, with the host of course only mapping back to one of them -- the outer one. This makes bijection.js more complicated and irregular than an actual bijection.

equate(g, h) had a test for early return, if the g and h were already "equated", i.e., were corresponding guest and host. But the equate test was written before the elaboration of bijection. In fact, it should only test whether this guest g maps to the host h, irrespective of whether h maps back to this g.

Unfortunately:

This failure was not tested by any async-flow test before it failed for a user of async-flow.

• This PR must add such a test before it becomes a candidate for merging

This PR should fix the failing test case https://github.com/Agoric/agoric-sdk/pull/9719/files#diff-cbd091458bbd51ddc6640a3783cdb340d7f2588cb7ff27b330ba228cfbd0e18dR64-R87 . Unfortunately, it does not. However, it now fails with different symptoms. Someone who understands #9719 better than I should investigate the nature of these new symptoms. If it still looks like it might be an async-flow problem, let me know!

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