Change the test that verify the query AST to use `EXPLAIN` instead

[mamcx]

Description of Changes

Stacked on top of #2075.

This removes the manual matches against the AST to use the new EXPLAIN support, to simplify the tests and make them easier to understand.

API and ABI breaking changes

None

Expected complexity level and risk

1

Testing

• [x] Most of the current tests pass.
• [ ] There are manual checks still for things we don't support

In concrete, we need support for multi-column indexes, index scans for ranges

[CLAassistant]

All committers have signed the CLA.