[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.