Fix Python test PYTHONPATH: use ENVIRONMENT_MODIFICATION instead of ENVIRONMENT (#584)

* Fix Python test PYTHONPATH: use ENVIRONMENT_MODIFICATION instead of ENVIRONMENT

The Python test environment setup in python/CMakeLists.txt uses
set_tests_properties(... ENVIRONMENT "PYTHONPATH=...") which overwrites
any PYTHONPATH set in the shell environment and bakes CMAKE_INSTALL_PREFIX
paths that don't exist at test time (before install).

Replace with ENVIRONMENT_MODIFICATION using path_list_prepend (CMake 3.22+)
to prepend CMAKE_BINARY_DIR/lib to PYTHONPATH and LD_LIBRARY_PATH at
test-execution time, preserving any paths already set in the environment.

Signed-off-by: Taylor Howard <taylorhoward@me.com>
Generated-by: Claude <noreply@anthropic.com>
(cherry picked from commit ca96b71)

Author: taylorhoward92

python/CMakeLists.txt

@@ -30,7 +30,17 @@ if (BUILD_TESTING AND NOT WIN32)
       add_test(NAME ${test}.py COMMAND
         "${Python3_EXECUTABLE}" "${CMAKE_SOURCE_DIR}/python/test/${test}.py")
     endif()
-    set(_env_vars "PYTHONPATH=${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/python/")
-    set_tests_properties(${test}.py PROPERTIES ENVIRONMENT "${_env_vars}")
+    set(_env_vars)
+    list(APPEND _env_vars "PYTHONPATH=path_list_prepend:${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/python/")
+    list(APPEND _env_vars "PYTHONPATH=path_list_prepend:${CMAKE_BINARY_DIR}/gz_msgs_gen/python/")
+    if(APPLE)
+      list(APPEND _env_vars "DYLD_LIBRARY_PATH=path_list_prepend:${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
+      list(APPEND _env_vars "DYLD_LIBRARY_PATH=path_list_prepend:${CMAKE_BINARY_DIR}/lib")
+    else()
+      list(APPEND _env_vars "LD_LIBRARY_PATH=path_list_prepend:${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
+      list(APPEND _env_vars "LD_LIBRARY_PATH=path_list_prepend:${CMAKE_BINARY_DIR}/lib")
+    endif()
+    set_tests_properties(${test}.py PROPERTIES
+      ENVIRONMENT_MODIFICATION "${_env_vars}")
   endforeach()
 endif()