test: replace add_date() calls in TPC-H queries with Ibis function calls

[ianmcook]

I was working with dates in Ibis and noticed that the TPC-H queries in this repo do literal date +/- interval math using a utility function that uses Python's datetime and dateutil. I reimplemented that all here using ibis.date() +/- ibis.interval().

In 31bb61f I added a bunch of .cast("date") but I'm not sure whether they are needed. It was super unclear to me whether the date columns are stored as dates or as strings.

[ianmcook]

I'll look at the test failures and make some changes

[gforsyth]

I think you can remove the cast but keep the calls to ibis.date(DATE)

The cast would be a no-op except that the initial sqlite schema has those dates as non-nullable and we default to nullable in the casts, so you'd have to pass l_shipdate.cast('date', nullable=False) everywhere and that would be a little noisy, I think.

[ianmcook]

@gforsyth should I make any changes here based on the additions in ibis-project/ibis#6003?

[gforsyth]

@ianmcook -- so I think the only things we can do here are:

1. Nothing / wait
2. Change the sqlite CI job to run on ibis master instead of the latest release.

But that will mean that ibis-master will be a defacto requirement of running this helper with sqlite and I'm not sure we want to do that.

[ianmcook]

Waiting seems fine to me. This is not urgent at all.