ESQL: Add abstract test cases for aggregation functions

[ivancea]

Scalar functions have the AbstractFunctionTestCase, which adds a ton of automatic testing to them. Aggregation functions currently have none. They just have some tests for the Aggregators themselves, but they aren't very "complete", and they don't cover the function as a whole (So an aggregator could not be being called at all).

We should have a similar test coverage for aggregators as we have for scalars.

• [x] Add base class for aggregation tests (https://github.com/elastic/elasticsearch/pull/110042)
• [x] Add tests for simple input+output
• [ ] Add tests for intermediate states
• [ ] Add test case generators for no rows

[elasticsearchmachine]

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