[C++][Compute] GroupBy: add unittests for individual components of hash group by

[asfimport]

Add unit tests directly verifying individual building blocks of hash group by:

• hash calculation
• bit utilities introduced by hash group by
• comparison
• hash table core
• encoding and decoding of column-oriented data to row-oriented format

Reporter: Michal Nowakiewicz / @michalursa

Related issues:

• [C++] Query engine umbrella issue (is a child of)

_{Note: This issue was originally created as ARROW-12723. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
@michalursa is this still valid?

[asfimport]

Neal Richardson / @nealrichardson:
This issue has been inactive for 3 months, so it has been unassigned and marked as unstarted. If you are still working on this, feel free to reassign yourself and resume progress.

[github-actions]

This issue has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this issue will be closed in 14 days. If this improvement is still desired but has no current owner, please add the 'Status: needs champion' label.