os/signal: tests: Adjust exit code expectation for Windows

On windows upon terminating signal reception exict code of the process
is always 3 instead of -signo. And so test_signal_all was failing as

        def test_signal_all():
            retcode, out, _ = _pyrun([dir_testprog + "/signal_test_all.py"], stdout=PIPE)
            assert b"ok (notify)"        in out
            assert b"ok (ignore)"        in out
            assert b"terminating ..."    in out
    >       assert retcode == -syscall.SIGTERM.signo
    E       assert 3 == -15
    E        +  where 15 = os.Signal(15).signo
    E        +    where os.Signal(15) = syscall.SIGTERM

golang/os/signal_test.py

@@ -203,7 +203,7 @@ def test_signal_all():
     assert b"ok (notify)"        in out
     assert b"ok (ignore)"        in out
     assert b"terminating ..."    in out
-    assert retcode == -syscall.SIGTERM.signo
+    assert retcode == (-syscall.SIGTERM.signo  if os.name != 'nt'  else 3)
 
 
 # test_stdlib_interop verifies that there is minimal compatibility in between