ESQL: Increase test coverage for INLINESTATS

[alex-spies]

• Tests with SORTs upstream from the INLINESTATS.
• More tests for all kind of shadowing scenarios (and double check what we already have)
• Tests with BUCKET in the BY-clause
• More cases with ROW, c.f. this comment
• Think of things that might have interesting interactions, like full text search (we had a bug with LOOKUP JOIN in the past, so that's not trivial).
• INLINESTATS with constants and foldables
• Think about what else might require additional tests, including invalid queries.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)