GH-45279: [C++][Compute] Move all Grouper tests to grouper_test.cc (#45280)

### What changes are included in this PR?

1. Move tests for the Grouper and RowSegmenter implementations to `arrow/compute/row/grouper_test.cc` where they belong (this also makes `acero/hash_aggregate_test.cc` slightly shorter)
2. Introduce a `arrow_compute_testing` object library to hold compute-related testing utilities
3. Remove duplicate definitions of `ExecBatchFromJSON` from Dataset and Acero
4. Make `arrow/compute/kernels/test_util.h` internal

### Are these changes tested?

Yes, by existing tests and CI.

### Are there any user-facing changes?

No.

* GitHub Issue: #45279

Authored-by: Antoine Pitrou <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

Author: pitrou

cpp/src/arrow/acero/CMakeLists.txt

@@ -117,7 +117,7 @@ if(ARROW_TESTING)
   if(ARROW_WITH_OPENTELEMETRY)
     target_link_libraries(arrow_acero_testing PRIVATE ${ARROW_OPENTELEMETRY_LIBS})
   endif()
-  list(APPEND ARROW_ACERO_TEST_LINK_LIBS arrow_acero_testing)
+  list(APPEND ARROW_ACERO_TEST_LINK_LIBS arrow_acero_testing arrow_compute_testing)
 endif()
 # Only for hash_aggregate_test.cc.
 if(ARROW_USE_BOOST)

cpp/src/arrow/acero/aggregate_node.h

@@ -24,6 +24,7 @@
 
 #include "arrow/acero/visibility.h"
 #include "arrow/compute/api_aggregate.h"
+#include "arrow/compute/test_util_internal.h"
 #include "arrow/compute/type_fwd.h"
 #include "arrow/result.h"
 #include "arrow/type_fwd.h"

cpp/src/arrow/acero/aggregate_node_test.cc

@@ -24,6 +24,7 @@
 
 #include "arrow/acero/test_util_internal.h"
 #include "arrow/compute/api_aggregate.h"
+#include "arrow/compute/test_util_internal.h"
 #include "arrow/result.h"
 #include "arrow/table.h"
 #include "arrow/testing/gtest_util.h"
@@ -32,6 +33,8 @@
 
 namespace arrow {
 
+using compute::ExecBatchFromJSON;
+
 namespace acero {
 
 Result<std::shared_ptr<Table>> TableGroupBy(

cpp/src/arrow/acero/asof_join_node_test.cc

@@ -41,8 +41,9 @@
 #include "arrow/acero/util.h"
 #include "arrow/api.h"
 #include "arrow/compute/api_scalar.h"
-#include "arrow/compute/kernels/test_util.h"
+#include "arrow/compute/cast.h"
 #include "arrow/compute/row/row_encoder_internal.h"
+#include "arrow/compute/test_util_internal.h"
 #include "arrow/testing/gtest_util.h"
 #include "arrow/testing/matchers.h"
 #include "arrow/testing/random.h"
@@ -67,6 +68,7 @@ namespace arrow {
 
 using compute::Cast;
 using compute::Divide;
+using compute::ExecBatchFromJSON;
 using compute::Multiply;
 using compute::Subtract;