factor out encoding dependent json/jsonb regression tests

I'm rather tired of having to update two sets of output files every time
we change the json and jsonb regression tests. Also, other committers
occasionally fall over this trap and fail to update json_1.out and
jsonb_1.out, as do almost all other patch authors. I propose that we
remove those tests to quite small json_utf8.sql and jsonb_utf8.sql
(which will each have 2 output files) and that will allow us to get rid
of json_1.out and jsonb_1.out. I'd like to backpatch this to 9.4 for
jsonb and 9.2 for json. Alternatively, for >= 9.4 we could just add the
jsonb encoding-specific tests to json_utf8.sql, to avoid a greater
proliferation of tests.

Thoughts?

cheers

andrew