test: Add a new integration test with minor dialogs

This patch adds a new integration test, which executes various small
dialogs, to cover corner cases that are not well covered (according to
our coverage report).

For example, "EHLO" without domain, or invalid DATA.

While we could do them via Go tests, this way is more realistic, and the
tests are easier to write.

test/t-12-minor_dialogs/unknown_command.cmy

@@ -0,0 +1,9 @@
+
+c tcp_connect localhost:1025
+
+c <~ 220
+c -> EHLO localhost
+c <... 250 HELP
+c -> WHATISTHIS
+c <- 500 unknown command
+