[FEA] Move cudf::test::print out of test into a public API

[ttnghia]

With the growth of libcudf development, there is also an increasing number of debugging tasks. Printing a column content is one of the useful debugging approaches. In cudf, we can easily call cudf::test::print for that purpose, but the downstream applications don't have access to such functionality in the C++ test module.

I propose to change the function cudf::test::print into a public API cudf::print(column_view const&) or cudf::to_string(column_view const&).

[davidwendt]

I would rather consider a cmake compile option that links the libcudftestutil.a with libcudf.so such that you can use cudf::test::print within libcudf functions temporarily for development.
There is a bunch of code in cudf::test::print and I fear moving it to cudf::print would generate too many distracting requirements including scalability and performance. I feel it is a perfectly suited for debugging small test cases and would prefer it be maintained in cudf::test.

Another option is to use the CSV writer with a data_sink that takes a std::vector<char>. The CSV writer contains alot of specialized code to convert columns to strings and I would not want to duplicate it unnecessarily.

[ttnghia]

I have another idea: continue to keep cudf::test::print in the test framework but make that utility function independently compilable in a separate TU. When somebody needs to use it, they can manually change the cmake file to include such TU directly into libcudf.

See #13720.