[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.

# type attribute might already be there if migrating from
# version < 3.5 to version >= 3.5.3, BaseTransition being added
# in bootstrap_migration
if versions_map['cubicweb'][0] >= (3, 5, 0):
    add_attribute('BaseTransition', 'type')
    sync_schema_props_perms('state_of')
    sync_schema_props_perms('transition_of')