[FEA] Extend tests to ensure no memory is allocated outside of RMM

**Is your feature request related to a problem? Please describe.**

It is important to ensure that all device memory allocated inside of cuDF functions is done through RMM.

It is easy to overlook this, e.g., by forgetting to pass the `rmm::exec_policy` to a Thrust algorithm that allocates temporary memory.

**Describe the solution you'd like**

It would be fairly easy to add this to our CI testing by writing a LD_PRELOAD library that overloads `cudaMalloc` to throw an error if it is called more than once.

This would ensure that there is only a single `cudaMalloc` call for the pool allocation.

There are some things to be aware of with this solution:
- We'd need to ensure the pool is sized such that it won't need to grow for the tests
- It would assume we're using cudaMalloc as the upstream resource for the pool (and not cudaMallocManaged)