Add a test for the format of the server address

In this commit I'm adding a test which ensures that the Server address
is a 2-tuple of (host, port), with the host being a string and the port
being a positive integer. This format is expected by existing code.

Author: diazona

tests/test_smtp.py

@@ -39,6 +39,13 @@ def test_smtpserver_funcarg(smtpserver):
     assert smtpserver.accepting and smtpserver.addr
 
 
+def test_smtpserver_addr(smtpserver):
+    host, port = smtpserver.addr
+    assert isinstance(host, str)
+    assert isinstance(port, int)
+    assert port > 0
+
+
 def test_server_is_killed(smtpserver):
     assert smtpserver.is_alive()
     smtpserver.stop()