Fix groupby gtests coded in namespace cudf::test

[davidwendt]

Description

Fixes GROUPBY_TEST gtests source files coded in namespace cudf::test
Additional creates a groupby_test_util.cpp for utilities rather than inlining the utilities into all 33 source files.
The groupby tests for aggregations std and var are also not compiled for Debug builds.
This changes those to compile but be runtime disabled to make them more noticeable in a Debug test run.

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.

[davidwendt]

I know there are a lot of files here. They are mostly namespace changes. Here are some exceptions:

• groupby_test_util.cpp: new file for implementing functions declared in groupy_test_util.hpp
• groupby_test_util.hpp: definitions moved to new groupby_test_util.cpp
• tests/gtest/replace/replace_nulls_tests.cpp: had unneeded include to groupby_test_util.hpp
• std_tests.cpp and var_tests.cpp: tests are not disable instead of compiled out in a Debug build

[vuule]

What a PR. Were you able to automate at least some of these changes?

[davidwendt]

/merge