[FEA] Remove mr from test fixture once we require CUDA 12

**Is your feature request related to a problem? Please describe.**
The base fixture used by our gtests holds an mr for using in tests. However, this mr is just cudf's own default mr, so this mr doesn't serve much purpose.

**Describe the solution you'd like**
We should remove this mr from the fixture.

**Additional context**
The removal was attempted in https://github.com/rapidsai/cudf/pull/14075, but it uncovered what appears to be a compiler bug that only manifested on arm machines using CUDA 11. We were unable to determine exactly what caused the compiler error. Since the issue is specific to CUDA 11, the change can be made once we drop support for CUDA 11 and CUDA 12 becomes the minimum required version.