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 <[email protected]>
Signed-off-by: David Li <[email protected]>

cpp/CMakeLists.txt

@@ -328,7 +328,6 @@ if(ARROW_BUILD_BENCHMARKS
    OR ARROW_BUILD_TESTS
    OR ARROW_BUILD_INTEGRATION
    OR ARROW_FUZZING)
-  set(ARROW_JSON ON)
   set(ARROW_TESTING ON)
 endif()
 
@@ -366,6 +365,10 @@ if(ARROW_SKYHOOK)
   set(ARROW_WITH_SNAPPY ON)
 endif()
 
+if(ARROW_TESTING)
+  set(ARROW_JSON ON)
+endif()
+
 if(ARROW_DATASET)
   set(ARROW_COMPUTE ON)
   set(ARROW_FILESYSTEM ON)