testsuite: Fix introspection test on Windows

testsuite: Fix introspection test on Windows

...when we are using Python 3.8.x or later.  Python 3.8.x or later on Windows
require one to call os.add_dll_directory() on every directory that contains
dependent non-system DLLs of a module that are not bundled/installed with the
module.

Since we are very likely running programs that rely on dependent items in
%PATH%, make things easier for people by calling os.add_dll_directory() on
all the valid paths in %PATH% in api.py, so that the test will run
successfully on Windows with Python 3.8.x or later.