Fix lists and structs gtests coded in namespace cudf::test (#11956)

Fixes structs and lists gtests source files coded in namespace `cudf::test`
These are the only 2 problem files for this in `cpp/tests/structs` and `cpp/tests/lists` and so will make those two directories complete. 
No function or test has changed just the source code reworked per namespaces.

Reference #11734

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

Approvers:
  - Vukasin Milovanovic (https://github.com/vuule)
  - Nghia Truong (https://github.com/ttnghia)

URL: #11956