test: move tpc-h testing setup into ibis

[cpcloud]

This PR moves our TPC-H testing into ibis.

Up to now, we've been doing tests with https://github.com/ibis-project/tpc-queries, which was usually fine.

However, during some development of #6996 the tpc-queries job started failing
and I moved TPC-H into ibis for faster debugging cycles.

This PR is the result of that work.

It turns out that for the other backend I added here (Snowflake) we're failing
three tpc-h tests that involve correlated subqueries (surprise!).

We can incrementally add support for more backends by:

1. Setting the appropriate supports_tpch flag in the backend's TestConf class.

2. Defining a few methods on the backend's TestConf class:

   class TestConf:
       ...
       def load_tpch(self) -> None:
           """Load TPC-H data into the current database
   
           Used in the DuckDB backend to run CALL dbgen(...)
           """
   
       def _tpch_table(self, name: str) -> ir.Table:
            """Return a table expression for the given TPC-H table
   
            Used in the Snowflake backend to access TPC-H tables, which
            reside in a different database than the one we're testing in
            """
   
       def _transform_tpch_sql(self, parsed: sg.exp.Expression):
            """Apply any semantics-preserving transformations to the parsed SQL.
   
            Used in the Snowflake backend to attach the database and schema to
            every leaf table
            """

[cpcloud]

Backend tests are passing:

…/ibis on  tpch-tests is 📦 v6.1.0 via 🐍 v3.10.12 via ❄️  impure (ibis-3.10.12-env)
❯ pytest -m 'bigquery or snowflake' --snapshot-update --allow-snapshot-deletion -n auto -q
bringing up nodes...
....xxxx...x.......xx.....x...x.....sxxxx.x..xxxxx......x...xx.xx.xssssx.x.x.xx.xx.xxsss.......x.x...xxx.sssss.x.x..x...x...x....ss.x..xx...x.....x....x.x....x...x...................x...x.x. [  6%]
..x.x......x......xxx......x....x.........xxxxxxxxxxxx.....xxxxx.x.xxxxx.xxxxxxx.xx..xx.xx.xs.sssx....xx.x..x.x.x.......x....x.xx.x.x.x.ss........x.x.......x.x.....x.x.........ss............ [ 13%]
.x.......x....sxxxxxxxx.xxxx..xxxxx.xx.xx...xxx....x...........x..x.........ss......x.........x.........x...x...x.....x..x......................s..x.x..............x..............x.......... [ 20%]
..x............x.....xx.......x...x................x.x...........x.............x.x..x..........xxx..........x..................x....x....x...........x......x...............x........x.....x.. [ 27%]
......................xx.....................................x........x...x.........xxx.....x....x.s.x......................x.............xx..xx.xxxxs.........x.x......x...x.x.............x. [ 33%]
.....x.........x...........................................x.....x........x..........x......x..x...x....x....xx......x...x........x.xx.........x...................x...x...xx......x.x..xx.... [ 40%]
..........................x..................................x......x.........xx........xx.....x....x.........x.........x...............x.x......x.x..........x....................x.......... [ 47%]
x..xx...x..............x...s......x.xx.....xx.....x................x...x....x.......................x.x.x.x..x..x....xxx...x.x......x...........x...x............x...x.....x....x..x.....x..x. [ 54%]
x....x.x...x...x............x.xxx..x......x...xx...x......x......x......x.x...x....x........xx.xxx............x.x...x.....x..x.......x........x..x...x.....xxx.x.....xx...x.....x..xx......... [ 60%]
.....x....x...x.......xx......x..x......x..............x.x.x...xx.x.x.....x.........xx........xxx.......x.x...........ssss.....s.x............x.......x...x......x........xx.....x......xx..x. [ 67%]
.xx........x...xx...x..x..........s............x......xx.......x.......x.............................x.x.....................x.............x....xx.x.xx..x.x.x.x.............................. [ 74%]
................................................x........................................................x..........xx............................................................s.........x. [ 81%]
.........x....x....x.........x.............x.x.......x......x...x...........................x..........x.........................x.x..........................xx.x.........x.....x............ [ 88%]
..x........................x..x.x...x..x..x....x...x.x........................................................................................................................................ [ 94%]
...................................................x..x..............................................................xxx.x.....s...........x.x....                                             [100%]
2335 passed, 38 skipped, 433 xfailed in 471.84s (0:07:51)

[gforsyth]