Test: Get the smtp test server using os.getenv() (#117979)

The smtp test server can be set via CPYTHON_TEST_SMTP_SERVER environment variable.
If not set, it uses the default value smtp.gmail.com
This is needed because the network I'm on filters access to
smtp.gmail.com resulting in a failing test.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>