Improve regression test coverage for src/backend/tsearch/spell.c.

Improve regression test coverage for src/backend/tsearch/spell.c.

In passing, throw an error if the AF count is too small, rather than
just silently discarding extra affix entries.

Note that the new regression test cases require installing the
updated src/backend/tsearch/dicts files.

Arthur Zakirov

Discussion: https://postgr.es/m/20180413113447.GA32474@zakirov.localdomain