Clarify test code.

"SMTPUTF8" was used as a standin for "an extension the server does not
support", which did no harm since the test server definitely didn't insert
that, but confused me for a moment, so this commit changes the name of the
unsupported extension to DOES-NOT-EXIST.

test/net/smtp/test_smtp.rb

@@ -76,13 +76,13 @@ def test_server_capabilities
         smtp = Net::SMTP.start('localhost', port, starttls: false)
         assert_equal({"STARTTLS"=>[], "AUTH"=>["PLAIN"]}, smtp.capabilities)
         assert_equal(true, smtp.capable?('STARTTLS'))
-        assert_equal(false, smtp.capable?('SMTPUTF8'))
+        assert_equal(false, smtp.capable?('DOES-NOT-EXIST'))
       else
         port = fake_server_start
         smtp = Net::SMTP.start('localhost', port, starttls: false)
         assert_equal({"AUTH"=>["PLAIN"]}, smtp.capabilities)
         assert_equal(false, smtp.capable?('STARTTLS'))
-        assert_equal(false, smtp.capable?('SMTPUTF8'))
+        assert_equal(false, smtp.capable?('DOES-NOT-EXIST'))
       end
       smtp.finish
     end