GH-47659: [C++] Fix Arrow Flight Testing's unresolved external symbol error (#47660)

### Rationale for this change

Fixes issue at #47659

### What changes are included in this PR?

Include add gmock as a shared private link library to `arrow_flight_testing`

### Are these changes tested?

Build for `arrow_flight_testing` succeeds on my Windows environment 

### Are there any user-facing changes?
No
* GitHub Issue: #47659

Authored-by: Alina (Xi) Li <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: alinaliBQ

cpp/src/arrow/flight/CMakeLists.txt

@@ -284,7 +284,9 @@ if(ARROW_TESTING)
          ArrowTesting::arrow_testing_static)
   endif()
   list(APPEND ARROW_FLIGHT_TESTING_SHARED_LINK_LIBS ${ARROW_FLIGHT_TEST_INTERFACE_LIBS})
+  list(APPEND ARROW_FLIGHT_TESTING_SHARED_LINK_LIBS ${ARROW_GTEST_GMOCK})
   list(APPEND ARROW_FLIGHT_TESTING_STATIC_LINK_LIBS ${ARROW_FLIGHT_TEST_INTERFACE_LIBS})
+  list(APPEND ARROW_FLIGHT_TESTING_STATIC_LINK_LIBS ${ARROW_GTEST_GMOCK})
   add_arrow_lib(arrow_flight_testing
                 CMAKE_PACKAGE_NAME
                 ArrowFlightTesting

cpp/src/arrow/flight/test_util.cc

@@ -25,6 +25,7 @@
 // We need Windows fixes before including Boost
 #include "arrow/util/windows_compatibility.h"
 
+#include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
 #include "arrow/array.h"
@@ -105,6 +106,20 @@ arrow::Result<FlightPayload> NumberingStream::Next() {
   return payload;
 }
 
+void AssertEqual(const FlightInfo& expected, const FlightInfo& actual) {
+  ipc::DictionaryMemo expected_memo;
+  ipc::DictionaryMemo actual_memo;
+  ASSERT_OK_AND_ASSIGN(auto ex_schema, expected.GetSchema(&expected_memo));
+  ASSERT_OK_AND_ASSIGN(auto actual_schema, actual.GetSchema(&actual_memo));
+
+  AssertSchemaEqual(*ex_schema, *actual_schema);
+  ASSERT_EQ(expected.total_records(), actual.total_records());
+  ASSERT_EQ(expected.total_bytes(), actual.total_bytes());
+
+  ASSERT_EQ(expected.descriptor(), actual.descriptor());
+  ASSERT_THAT(actual.endpoints(), ::testing::ContainerEq(expected.endpoints()));
+}
+
 std::shared_ptr<Schema> ExampleIntSchema() {
   auto f0 = field("f0", int8());
   auto f1 = field("f1", uint8());

cpp/src/arrow/flight/test_util.h

@@ -17,7 +17,6 @@
 
 #pragma once
 
-#include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
 #include <cstdint>
@@ -42,20 +41,8 @@ namespace flight {
 
 // ----------------------------------------------------------------------
 // Helpers to compare values for equality
-
-inline void AssertEqual(const FlightInfo& expected, const FlightInfo& actual) {
-  ipc::DictionaryMemo expected_memo;
-  ipc::DictionaryMemo actual_memo;
-  ASSERT_OK_AND_ASSIGN(auto ex_schema, expected.GetSchema(&expected_memo));
-  ASSERT_OK_AND_ASSIGN(auto actual_schema, actual.GetSchema(&actual_memo));
-
-  AssertSchemaEqual(*ex_schema, *actual_schema);
-  ASSERT_EQ(expected.total_records(), actual.total_records());
-  ASSERT_EQ(expected.total_bytes(), actual.total_bytes());
-
-  ASSERT_EQ(expected.descriptor(), actual.descriptor());
-  ASSERT_THAT(actual.endpoints(), ::testing::ContainerEq(expected.endpoints()));
-}
+ARROW_FLIGHT_EXPORT
+void AssertEqual(const FlightInfo& expected, const FlightInfo& actual);
 
 // ----------------------------------------------------------------------
 // Fixture to use for running test servers