refactor(planning_test_utils): separate hpp and cpp (#7082)

Signed-off-by: Muhammad Zulfaqar Azmi <zulfaqar.azmi@tier4.jp>

Author: zulfaqar-azmi-t4

planning/planning_test_utils/CMakeLists.txt

@@ -4,6 +4,9 @@ project(planning_test_utils)
 find_package(autoware_cmake REQUIRED)
 autoware_package()
 
+ament_auto_add_library(planning_test_utils SHARED
+  src/planning_test_utils.cpp)
+
 ament_auto_add_library(mock_data_parser SHARED
   src/mock_data_parser.cpp)