[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.
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/shared/data usr/share/cubicweb/cubes/shared
debian/tmp/usr/share/cubicweb/cubes/shared/wdoc usr/share/cubicweb/cubes/shared