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ARROW-12500: [C++][Datasets] Ensure better test coverage of Dataset file formats

This unifies (most of) the tests across Parquet, Feather, and CSV (with carve-outs for particular cases). In particular, this means all formats are now tested in conjunction with async/sync and serial/threaded scanners. Also, a set of common file format tests were refactored out of the individual tests and centralized.

Closes #10134 from lidavidm/arrow-12500

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>