GH-45279: [C++][Compute] Move all Grouper tests to grouper_test.cc

[pitrou]

What changes are included in this PR?

1. Move tests for the Grouper and RowSegmenter implementations to arrow/compute/row/grouper_test.cc where they belong (this also makes acero/hash_aggregate_test.cc slightly shorter)
2. Introduce a arrow_compute_testing object library to hold compute-related testing utilities
3. Remove duplicate definitions of ExecBatchFromJSON from Dataset and Acero
4. Make arrow/compute/kernels/test_util.h internal

Are these changes tested?

Yes, by existing tests and CI.

Are there any user-facing changes?

No.

• GitHub Issue: [C++][Compute] Move Grouper tests into grouper_test.cc #45279

[github-actions]

⚠️ GitHub issue #45279 has been automatically assigned in GitHub to PR creator.

[zanmato1984]

Wow, really nice refinement! Many things in it are what I have always wanted. Cheers!

Is it ready for review now or do I have to wait until it is no longer in draft?

[pitrou]

@zanmato1984 I was just waiting to fix CI, but yes, it's ready for review now.

[pitrou]

(of course, most of the changes here are just code being moved around)

[pitrou]

I would ideally like to merge this soon as I've also started to work on #45269

[zanmato1984]

LGTM with several nits.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit df82d4c.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 2 possible false positives for unstable benchmarks that are known to sometimes produce them.