test facility: find a more ergonomic way to build executors

[xxchan]

Updated: we may want to fix this style issue when enhancing test facilities.

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Original issue:

• Stream executors have a from_proto directory for all builders, but batch executors have them in each executor's file.
• Stream executors all have a builder, which is an empty struct, but batch executors have mixed style (personally I slightly prefer impl ExecutorBuilder for executors directly without Builder structs, since it's just a single method. We can use the Builder trait, but avoid the Builder struct)

[xxchan]

Found the stream executor reorg PR #2289

And initial builder PR #462

BTW, Builder seems not that necessary here. We can discuss it offline.

Originally posted by @MrCroxx in #462 (review)

🤔

[liurenjie1024]

We can have sth like ExecutorFactory to build executor tree directly, without having to build from proto. This helps a lot in writing unittests.

[liurenjie1024]

This also applies to expression, plan, etc.

[MrCroxx]

Found the stream executor reorg PR #2289

And initial builder PR #462

BTW, Builder seems not that necessary here. We can discuss it offline.
Originally posted by @MrCroxx in #462 (review)

🤔

IIRC, the context is that when building stream executors, some executors needs some extra field but the others don't, then @cykbls01 introduced a builder trait to solve it. I think it's okay to remove it if it's not used anymore.

[xxchan]

We can have sth like ExecutorFactory to build executor tree directly, without having to build from proto. This helps a lot in writing unittests.

That's indeed desired to enhance DX and help improves test coverage.

Prior arts:

• refactor(stream): human-friendly StreamChunk literal for test #2089 introduced DSL to build chunks i.e., test data easier.
• test: snapshot testing for stream executors #9787 made writing test data one step easier -- auto generate test results. Writing test data should be generally painless now, but it still takes effort to build executor.
• test facility: build BoxedExpression from "pretty" representation of expression #7881 introduced DSL to build expression easier.

We may consider how to build executors easier as the next enhancement.

• maybe some builder pattern
• maybe some DSL
• maybe reuse some fe code (This may be possible with Consider using integration test instead of unit test #9878)