[C++] Add test coverage for BitUtils::RoundDown

[asfimport]

Reporter: Francois Saint-Jacques / @fsaintjacques
Assignee: Francois Saint-Jacques / @fsaintjacques

PRs and other links:

• GitHub Pull Request #3646

_{Note: This issue was originally created as ARROW-4529. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 3646
#3646