Add regression test checking combinations of (object,backend_type,context) in pg_stat_io

Hi all,

While working on I/O statistics, I have noticed that it is not
complicated to break the set of rows returned by pg_stat_io if one is
not careful when updating pgstat_tracks_io_object().

Attached is a patch that I've found useful as a sanity check,
returning all the combinations supported for BackendType, IOContext
and IOObject, so as it is easily possible to evaluate if the
information returned is relevant.

Thoughts?
--
Michael