ARROW-6005: [C++] extend GetRecordBatchReader test to cover reading a single row group
ARROW-6005: [C++] extend GetRecordBatchReader test to cover reading a single row group

This adds a basic unittest for row group selection with the `GetRecordBatchReader` API.

Closes #5012 from hatemhelal/arrow-6005 and squashes the following commits:

c1ccb88a6 <Hatem Helal> extend GetRecordBatchReader test to cover reading a single row group

Authored-by: Hatem Helal <6770b6823b2402dc813fc3254c5869d67b3a1d13@mathworks.com>
Signed-off-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>

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