Fix reduction gtests coded in namespace cudf::test

[davidwendt]

Description

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

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-23.02@e67b94b). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@               Coverage Diff               @@
##             branch-23.02   #12257   +/-   ##
===============================================
  Coverage                ?   88.20%           
===============================================
  Files                   ?      137           
  Lines                   ?    22653           
  Branches                ?        0           
===============================================
  Hits                    ?    19981           
  Misses                  ?     2672           
  Partials                ?        0           

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[davidwendt]

@gpucibot merge