Failing GIN type confusion test

Currently, the default GIN opclass thinks that the exists operator can
be used to find Jsonb datums with JSON arrays that have only non-string
elements, on the basis of the textual storage format of those non-string
elements accidentally matching (i.e. being bitwise equal) to what would
be expected for an actual string element.  This is bogus, in that it
does not comport with the general definition of Jsonb existence (that
is, the behavior of the ?/exists operator, and the numerous slight
variants of same).

The likely fix here is to represent JsonbValue type directly in the
first byte of the serialized text representation of the opclass (i.e.
in addition to keylement and value flags, there'd be a JsonbValue type
flag), but just commit this test for now.