[3.13] gh-135966: Modify iOS testbed to make app_packages a site directory (GH-135967) (#136013)

The iOS testbed now treats the app_packages folder as a site folder. This ensures it is
on the path, but also ensures any .pth files are processed on app startup.
(cherry picked from commit b38810bab76c11ea09260a817b3354aebc2af580)