Cover the password prompt case for `SSLContext.load_cert_chain` with a test

[donBarbos]

@picnixz found a case that is not covered by the tests: when we specify a cert and a password-protected file, but no password. This behavior can be reproduced in the following way (password is somepass):

>>> import ssl
>>> context = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
>>> context.load_cert_chain(certfile="./Lib/test/certdata/keycert.passwd.pem")
Enter PEM pass phrase:

Documentation says that in this case the password should be requested interactively

If the password argument is not specified and a password is required, OpenSSL’s built-in password prompting mechanism will be used to interactively prompt the user for a password.

We have great tests (./Lib/test/test_ssl) for the ssl module but unfortunately this case has no tests, so I suggest to add it

[picnixz]

Considering #129607 (comment), I'll forward my reply (#129607 (comment)):

[...] if you think it's fine not to cover this, I'm also fine (we test everything but the interactive prompt; but I guess that the interactive prompt is actually populating the context so we don't need to do anything on our side).

So maybe this is not needed. Sorry for making you create this issue.