[#1838] Add tests for current index creation code
[#1838] Create FTS indexes on each textual field

We create those indexes at the same time we're indexing `_full_text`. This is
needed to allow full-text searches on specific columns, and not only on the
entire row.

For the test, unfortunately I couldn't test that a FTS index was created for
each specific field, but instead rely on the number of indexes, because as the
index is not on the column itself, but on the return value of `to_tsvector`, we
don't have that information available.

There're still issues with this code as we're using English for default (and
there's no way to overwrite that).