Fix reduction gtests coded in namespace cudf::test (#12257)

Fixes reduction gtests source files coded in namespace `cudf::test`
No function or test has changed just the source code reworked per namespaces.

Fixing this ahead of any changes for #10432 
Reference #11734

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Bradley Dice (https://github.com/bdice)
  - MithunR (https://github.com/mythrocks)

URL: #12257