bpo-34783: Disable test_nonexisting_script for macOS framework builds (GH-9831)

With macOS framework builds, test case test_nonexisting_script in
test_nonexisting_script fails because the test case assumes that
the file name in sys.executable will appear in the error message.
For macOS framework builds, sys.executable is the file name of the
stub launcher and its file name bears no relationship to the file
name of the actual python executable.  For now, skip the test in
this case.