ES|QL: Improve generative tests for FORK

[ioanatia]

In #129415 we added very basic support in generative test for FORK.

The FORK generator only appends a FORK (WHERE true) (WHERE true) (WHERE true) ... (WHERE true) | EVAL _fork = <random fork ID> | DROP _fork to the existing query. We then test that we are getting the same results we got before we called ForkGenerator.generate.

Now that FORK branches should be able to support all commands (except FORK in FORK), we can improve these tests.
We should be able to generate different branches for FORK and then test that we are getting results back.

[elasticsearchmachine]

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