Add more tests for getpass

This is something I thought of when working #130496, I noticed that some tests were missing.
Test Lib/test/test_getpass.py only has three classes: GetpassGetuserTest, GetpassRawinputTest, UnixGetpassTest.

This clearly doesn't cover all cases, I think we should add at least WinGetpassTest and check that both implementations (unix_getpass and win_getpass) work the same.