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PARQUET-1569: [C++] Consolidate shared unit testing header files

This makes the codebase slightly structurally simpler. Note there is still some code duplication around random number generation that would be good to simplify in the future

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

Closes #4252 from wesm/parquet-consolidate-test-headers and squashes the following commits:

060ebfdb2 <Wes McKinney> Fix Windows conversion warning
4a2ddc605 <Wes McKinney> Reuse some random number generation code
dd46bb403 <Wes McKinney> Combine Parquet testing headers