[C++] Implement AssertDatumEquals

[asfimport]

Aggregate tests could benefit from this.

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

PRs and other links:

• GitHub Pull Request #3875

_{Note: This issue was originally created as ARROW-4673. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
[~emkornfield@gmail.com] replied on the mailing list "It might be nice to do this as a Gmock matcher instead of a separate macro"

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
We will need ToString methods for every sub-objects to use Matchers. A second note on this, while debugging it's very handy if the Matcher can pin-point the exact reason, e.g. row, where the miss-match happens.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3875
#3875