arrow

MINOR: [C++] ARROW_TESTING implies ARROW_JSON (#14054)

Otherwise a minimal build fails with errors like

```
CMake Error at cmake_modules/BuildUtils.cmake:272 (target_link_libraries):
  Target "arrow_testing_objlib" links to:

    rapidjson::rapidjson

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Call Stack (most recent call first):
  src/arrow/CMakeLists.txt:653 (add_arrow_lib)
```

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>