Write regression test guidelines

It's hard to remember all the rules that make for good regression tests. We should add a document to the wiki that succinctly describes the rules to follow.

Some basic ideas:

If applicable, test whether a new feature:
- shows the expected error messages when used incorrectly
- works with prepared statements
- works in a transaction block
- works in the same transaction as other commands (e.g. DDL)
- works with reference tables
- works with MX
- locks tables and shards appropriately
- ...

(don't need to have all of these all the time, but just good to remember pitfalls)

Do *not*:
- Show output that depends on prior or concurrent tests
- Use COPY with files
- Show unordered SELECT results
- Show timing
- Show shard IDs
- EXPLAIN without (COSTS OFF)
- Use the same tables across different tests
- Use DEBUG output unless
- ...

(unless absolutely necessary, e.g. EXPLAIN always shows shard IDs)