[VL] Separate cpp test utils from the utils directory

### Description

Currently the utils for testing/benchmarks are located under cpp/velox/utils, and are built together when testings are not built. We should move them to a sub folder under tests and only build if `BUILD_TESTS=ON` or `BUILD_BENCHMARKS=ON`

### Gluten version

None

Start by examining cpp/velox/utils and the build configuration controlling test and benchmark targets. Trace how these utilities are currently included, then verify that they live under a tests subfolder and are built only when BUILD_TESTS=ON or BUILD_BENCHMARKS=ON.

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