[test/web] fix invisibly bogus test (prepares #2755994) The test was wrong but that was cancelled out by a cache effect and fuzzy naming. Wiping the entity caches restores sanity: the choices list are the same before and after the SET. Also field.choices uses entity.unrelated but always returns related + unrelated elements. Hence `choice` replaces `unrelated` where it makes sense. AssertIn is used in place of AssertTrue.

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