[tests] Allow clockskew in server context expiration time check

krb5 in newer versions adds the clockskew to the server context
lifetime, which is permitted by RFC.  Allow this behavior in
test_high_level.py.

Author: frozencemetery

gssapi/tests/test_high_level.py

@@ -651,7 +651,7 @@ def test_complete_on_partially_completed(self):
     def test_initiate_accept_steps(self):
         client_ctx, server_ctx = self._create_completed_contexts()
 
-        server_ctx.lifetime.should_be_at_most(400)
+        server_ctx.lifetime.should_be_at_most(400 + 300) # clockskew
         server_ctx.initiator_name.should_be(client_ctx.initiator_name)
         server_ctx.mech.should_be_a(gb.OID)
         server_ctx.actual_flags.should_be_a(gb.IntEnumFlagSet)