xform: reorganize tests to match the organization of the rules

This commit moves around (and makes some minor necessary changes) to
exploration tests so that tests for rules in `foo.opt` can be found in
`testdata/foo`. Most of the changes were related to zigzag join rules
which moved from `join` to `select`, and some tests for
`GenerateConstrainedScans` that have lived in `scan` for years.

The goal of this change is to make it easier to find tests for a
specific rule and to set a precedent for future tests.

Release note: None