Fixed #30806 -- Prevented tox tests hanging on macOS.

MacOS users may need to set OBJC_DISABLE_INITIALIZE_FORK_SAFETY=TRUE
(#27086) to prevent tests from hanging.  If set, also pass the environment
variable to `tox`.

tox.ini

@@ -17,7 +17,8 @@ basepython = python3
 
 [testenv]
 usedevelop = true
-passenv = DJANGO_SETTINGS_MODULE PYTHONPATH HOME DISPLAY
+# OBJC_DISABLE_INITIALIZE_FORK_SAFETY fixes hung tests for MacOS users. (#30806)
+passenv = DJANGO_SETTINGS_MODULE PYTHONPATH HOME DISPLAY OBJC_DISABLE_INITIALIZE_FORK_SAFETY
 setenv =
     PYTHONDONTWRITEBYTECODE=1
 deps =