REST tests fetching fields with synthetic _source (#89888)

This adds some paranoid REST layer tests for fetching field values with
synthetic _source. Mostly, everything just works. But there are a few
cases that work with regular source and not synthetic. This PR just adds
those tests and asserts that those actions fail with a `TODO` that they
should pass. We'll fix them in follow up changes. One by one by one.

The failures are: * `keyword` fields without doc values try to load from
the non-existent stored `_source`. * `text` fields for loaded via the
`field` API try to load from the non-existent stored `_source`. *
`match_only_text` field access in scripts tries to load from the
non-existent stored `_source`. All script access regardless of how.

![image](https://user-images.githubusercontent.com/215970/188951824-808fe047-baae-4984-b6a7-d3b05d89869e.png)