Separate header for utility functions made specifically for tests.

[mmelnich]

Riley says:

For now I just suggest you stick these helper utils in rl_util.hh. There are only three or four definitions in this file. Then you can open an issue on the RandBLAS repo saying that we need to expose more utilities for debugging. Right now most of these utilities are in the tests/ folder of the RandBLAS repo, which is not actually packaged with a RandBLAS installation. Once we decide how to handle for RandBLAS we can handle in the same way for RandLAPACK.

[rileyjmurray]

Please open on the RandBLAS repo.