[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.