CXX-2753 Move test files into test subdirectory

[eramongodb]

This PR is a followup to #1037 and the last expected PR related to CXX-2753. Verified by this patch.

As with the prior PRs, this PR does NOT affect binary or source compatibility. It only moves around test files and modifies CMake config and test code accordingly.

Specifically, the bsoncxx catch.hh and to_string.hh headers formerly located under lib/bsoncxx/v_noabi/bsoncxx/test_util is moved into the test subdirectory. These headers are only used by test code and are not required by the bsoncxx library itself. The mongocxx client_helpers component formerly under lib/mongocxx/v_noabi/mongocxx/test_util is moved into the test subdirectory for the same reason. This is also supported by the fact that all code that needed to be updated with test_util -> test are already under the test subdirectories for each project.

To support this relocation, the ${CMAKE_CURRENT_SOURCE_DIR}/../.. directory (which corresponds to <repo-root>/src) is added to the include directories of test targets. This is to allow inclusion of test headers via <bsoncxx/test/...> and <third_party/...>. This was seen as preferable to simply <header.(hpp|hh)> without any prefix. This technically also permits <bsoncxx/include/...>, <bsoncxx/lib/...>, and etc. in test code, but this is not an intended or supported pattern and should be rejected in future code reviews.

[kevinAlbs]

LGTM. Suggested removal of client_helpers_cpp variable.