arrow

ARROW-4504: [C++] Reduce number of C++ unit test executables from 128 to 82

This yields approximately 15% net improvement in build time when building with msbuild and `/MP` on a quad-core desktop. Parallel test execution times on Linux/macOS should be unaffected -- I took care to only combine executables where it made sense and where execution time was already very fast (100s of milliseconds or less)

Author: Wes McKinney <wesm+git@apache.org>

Closes #4442 from wesm/ARROW-4504 and squashes the following commits:

35b81f63a <Wes McKinney> Revert changes to io/CMakeLists.txt
19beb21b7 <Wes McKinney> Merge unit test objects to get down to 78 unit tests total